[FS#929] mt7620 abysmal wifi performance

[LEDE Bugs]

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

User who did this - pmgp (mgondium) 

Attached to Project - LEDE Project
Summary - mt7620 abysmal wifi performance
Task Type - Bug Report
Category - Base system
Status - Unconfirmed
Assigned To - 
Operating System - All
Severity - High
Priority - Very Low
Reported Version - Trunk
Due in Version - Undecided
Due Date - Undecided
Details - HT40 wifi throughput went from ~80 Mbps(LEDE 17.01.0) to ~3Mpbs (since LEDE 17.01.2 to trunk, 17.01.1 wouldn't build).
HT20 decreased from ~40 to ~10 Mbps.

device is archer C20i (mt7620a) on 2.4 GHz band, 5 Ghz is unsupported (mt7610E).

Log errors:

[    9.197616] ieee80211 phy0: rt2x00_set_rt: Info - RT chipset 6352, rev 0500 detected
[    9.205560] ieee80211 phy0: rt2x00_set_rf: Info - RF chipset 7620 detected
[  304.206295] ieee80211 phy0: rt2x00queue_flush_queue: Warning - Queue 2 failed to flush
[  304.278242] ieee80211 phy0: rt2x00queue_flush_queue: Warning - Queue 2 failed to flush
[  306.059378] ieee80211 phy0: rt2x00queue_write_tx_frame: Error - Arrived at non-free entry in the non-full queue 2

More information can be found at the following URL:
https://bugs.lede-project.org/index.php?do=details&task_id=929