ath9k_htc: Fix BSSID calculation

The BSSID/AID has to be set for the first associated station interface.
Subsequent interfaces may move out of assoc/disassoc status, in which
case, the BSSID has to be re-calculated from the available interfaces.
Also, ANI should be enabled or disabled based on the current opmode.

Signed-off-by: Sujith Manoharan <Sujith.Manoharan@atheros.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>