ath9k: Handle BSSID/AID for multiple interfaces

As of now bssid/aid is overridden with recently changed vif's
bss config. This may cause improper beacon updation due to
bssid/aid mismatch. On station mode, select an associated
sta vif as primary vif and configure that vif's bss into hw.
Update the primary vif on interface change and bss info change.

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