[FS#395] mac80211.sh: detect_mac80211 should check available channels

[LEDE Bugs]

A new Flyspray task has been opened.  Details are below. 

User who did this - Rafał Miłecki (rmilecki) 

Attached to Project - LEDE Project
Summary - mac80211.sh: detect_mac80211 should check available channels
Task Type - Bug Report
Category - Base system
Status - New
Assigned To - 
Operating System - All
Severity - Low
Priority - Medium
Reported Version - Trunk
Due in Version - Undecided
Due Date - Undecided
Details - This problem was reported by few Netgear R8000 users and I managed to reproduce it. It's related to the following code:
		vht_cap=$(iw phy "$dev" info | grep -c 'VHT Capabilities')
		cap_5ghz=$(iw phy "$dev" info | grep -c "Band 2")
		[ "$vht_cap" -gt 0 -a "$cap_5ghz" -gt 0 ] && {
			mode_band="a";
			channel="36"
			htmode="VHT80"
		}
===========Default htmode===========
The first problem is that even if hardware supports VHT80 it may be not available for every/any current channel. If channel 36 doesn't support VHT80, for better user experience, it shouldn't be set in /etc/config/wireless by default. Example:root at lede:/# iw phy phy2 info
Wiphy phy2
        max # scan SSIDs: 10
        max scan IEs length: 2048 bytes
        max # sched scan SSIDs: 0
        max # match sets: 0
        max # scan plans: 1
        max scan plan interval: -1
        max scan plan iterations: 0
        Retry short limit: 7
        Retry long limit: 4
        Coverage class: 0 (up to 0m)
        Device supports roaming.
        Available Antennas: TX 0 RX 0
        Supported interface modes:
                 * IBSS
                 * managed
                 * AP
                 * P2P-client
                 * P2P-GO
                 * P2P-device
        Band 2:
                Capabilities: 0x1062
                        HT20/HT40
                        Static SM Power Save
                        RX HT20 SGI
                        RX HT40 SGI
                        No RX STBC
                        Max AMSDU length: 3839 bytes
                        DSSS/CCK HT40
                Maximum RX AMPDU length 65535 bytes (exponent: 0x003)
                Minimum RX AMPDU time spacing: 16 usec (0x07)
                HT TX/RX MCS rate indexes supported: 0-23
                VHT Capabilities (0x0c025820):
                        Max MPDU length: 3895
                        Supported Channel Width: neither 160 nor 80+80
                        short GI (80 MHz)
                        SU Beamformer
                        SU Beamformee
                VHT RX MCS set:
                        1 streams: MCS 0-9
                        2 streams: MCS 0-9
                        3 streams: MCS 0-9
                        4 streams: not supported
                        5 streams: not supported
                        6 streams: not supported
                        7 streams: not supported
                        8 streams: not supported
                VHT RX highest supported: 0 Mbps
                VHT TX MCS set:
                        1 streams: MCS 0-9
                        2 streams: MCS 0-9
                        3 streams: MCS 0-9
                        4 streams: not supported
                        5 streams: not supported
                        6 streams: not supported
                        7 streams: not supported
                        8 streams: not supported
                VHT TX highest supported: 0 Mbps
                Frequencies:
                        * 5170 MHz [34] (disabled)
                        * 5180 MHz [36] (20.0 dBm)
                        * 5190 MHz [38] (disabled)
                        * 5200 MHz [40] (20.0 dBm)
                        * 5210 MHz [42] (disabled)
                        * 5220 MHz [44] (20.0 dBm)
                        * 5230 MHz [46] (disabled)
                        * 5240 MHz [48] (20.0 dBm)
                        * 5260 MHz [52] (20.0 dBm) (no IR, radar detection)
                        * 5280 MHz [56] (20.0 dBm) (no IR, radar detection)
                        * 5300 MHz [60] (20.0 dBm) (no IR, radar detection)
                        * 5320 MHz [64] (20.0 dBm) (no IR, radar detection)
                        * 5500 MHz [100] (20.0 dBm) (no IR, radar detection)
                        * 5520 MHz [104] (20.0 dBm) (no IR, radar detection)
                        * 5540 MHz [108] (20.0 dBm) (no IR, radar detection)
                        * 5560 MHz [112] (20.0 dBm) (no IR, radar detection)
                        * 5580 MHz [116] (20.0 dBm) (no IR, radar detection)
                        * 5600 MHz [120] (20.0 dBm) (no IR, radar detection)
                        * 5620 MHz [124] (20.0 dBm) (no IR, radar detection)
                        * 5640 MHz [128] (20.0 dBm) (no IR, radar detection)
                        * 5660 MHz [132] (20.0 dBm) (no IR, radar detection)
                        * 5680 MHz [136] (20.0 dBm) (no IR, radar detection)
                        * 5700 MHz [140] (20.0 dBm) (no IR, radar detection)
                        * 5720 MHz [144] (disabled)
                        * 5745 MHz [149] (disabled)
                        * 5765 MHz [153] (disabled)
                        * 5785 MHz [157] (disabled)
                        * 5805 MHz [161] (disabled)
                        * 5825 MHz [165] (disabled)
        valid interface combinations:
                 * #{ managed }