ath9k: optimize/fix ANI RSSI processing

ANI needs the RSSI average only in station mode, and only for tracking
the signal strength of beacons of the AP that it is connected to.
Adjust the code to track on the beacon RSSI, and store the average of that
in the ath_wiphy struct.
With these changes, we can get rid of this extra station lookup in the
rx path, which saves precious CPU cycles.

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