ath9k: enable RXing of beacons on STA/IBSS

This enables beacons to come through on STA/IBSS.
It should fix sporadic connection issues. Right now
mac80211 expect beacons so give it beacons.

Signed-off-by: Luis R. Rodriguez <lrodriguez@atheros.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>