ath9k_hw: clean up / fix ANI mode checks related to beacon RSSI

Beacon RSSI is only meaningful in station mode - in ad-hoc mode it
fluctuates, depending on which peer last sent a beacon, and in other
modes it is not set at all.
Fix places in ANI where the beacon RSSI is used to limit their use
to station mode only.

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