vht_capab reading 0x0 but `iw list` says otherwise

[James Sumners]

When I launch hostapd I see a line in the log that reads "hw vht capab:
0x0, conf vht capab: 0x0". That is accurate because I currently have no
`vht_capab` line defined in my configuration. However, if I do an `iw
list` on my device then I see the following list of capabilities:

```
VHT Capabilities (0x038071a0):
  Max MPDU length: 3895
  Supported Channel Width: neither 160 nor 80+80
  short GI (80 MHz)
  TX STBC
  SU Beamformee
VHT RX MCS set:
  1 streams: MCS 0-9
  2 streams: MCS 0-9
  4 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
  4 streams: not supported
  ...
  8 streams: not supported
VHT TX highest supported: 0 Mbps
```

When I set my hostapd config to have the line
`vht_capab=[SHORT-GI-80][TX-STBC-2BY1][SU-BEAMFORMEE]` then I get
an error that "SHORT-GI-80" is not supported. I can remove each
capability from the configuration directive and get the same sort of
result.

When I set `vht_oper_chwidth=1`, without setting `vht_capab`, then I
get an error:

```
Mode: IEEE 802.11g Channel: 4 Frequency: 2427 MHz
DFS chan_idx seems wrong; num-ch: 13 ch-no: 36 conf-ch-no: 4 11n: 1
sec-ch: 0 vht-oper-width: 1
// list of channels available - [1, 13]
```

If I set `vht_oper_chwidth=0`, still without `vht_capab` set, then I
get an error:

```
Mode: IEEE 802.11g Channel: 4 Frequency: 2427 MHz
DFS 0 channels required radar detection
Could not set channel for kernel driver
```

If I set `ieee80211ac=0` then hostapd starts up correctly (no other
changes to the configuration).

My card is an Intel 7260 that supports both AP and "AP/VLAN" modes with
one BSSID. I am using the nl80211 driver (i.e. `driver=nl80211` in
hostapd.conf).

Do you have any suggestions as to what could be wrong with my setup?
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