mac80211: sync some rt2x00 patches with wireless-next

mac80211: sync some rt2x00 patches with wireless-next

Some local patches have been sent to upstream and they are slightly
different from the upstream version. So it's better to replace them
to avoid conflicts with the new mac80211 backport driver. The
different parts have been merged into patch 996.

This commit also includes some additional fixes:
* Fix watchdog function.
* Improve MT7620 register initialization.
* Introduce DMA busy watchdog for rt2800.

P.S.
Sometimes rt2800 series chips may fall into a DMA busy state. The
tx queues become very slow and the client cannot connect to the AP.
Usually, We can see a lot of hostapd warnings at this point:
'hostapd: IEEE 802.11: did not acknowledge authentication response'

The DMA busy watchdog can help the driver automatically recover
from this abnormal state. By the way, setting higer 'cell_density'
and disabling 'disassoc_low_ack' can significantly reduce the
probability of the DMA busy.

Signed-off-by: Shiji Yang <yangshiji66@qq.com>