ath9k: reduce the likelihood of baseband hang check false positives

Since baseband hangs are rare, but the hang check function has a high
false positive rate in some situations, we need to add more reliable
indicators.

In AP mode we can use blocked beacon transmissions as an indicator,
they should be rare enough.

In station mode, we can skip the hang check entirely, since a true
hang will trigger beacon loss detection, and mac80211 will rescan,
which leads to a hw reset that will bring the hardware back to life.

To make this more reliable, we need to skip fast channel changes
if the hardware appears to be stuck.

Signed-off-by: Felix Fietkau <nbd@openwrt.org>
Signed-off-by: John W. Linville <linville@tuxdriver.com>