ath9k: validate for non-zero BSSID

before concluding that the recieved beacon is for us, let us make sure
that the BSSID is non-zero. when I configured ad-hoc mode as creator and
left it for some time without joining I found we recieved few frames whose
BSSID is zero, which we concluded wrongly as 'my_beacons'

Signed-off-by: Mohammed Shafi Shajakhan <mohammed@qca.qualcomm.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>