ath9k: configure beacons based on hw opmode

Current ath9k code does not handle beacon timers on opmode
specific. One such example is that a STA beacon config overwrites
already configured AP vif's beacon timers during scan.

On multi station vif case, configure beacon timers beased
on primary vif selected. This also helps while moving back
to single STA vif from multi STA vifs, where the power save
is enabled and hw has to be reconfigured with proper
beacon and bssid/aid. Otherwise connection poll will be triggered
so frequently due to beacon loss.

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