ath9k: Fix beacon generation on foreign channel

While leaving the oper channel, beacon generation is stopped
by mac80211 and beacon slots are marked as inactive.
During the scan, ath9k configures beacon timers
based on IEEE80211_CONF_OFFCHANNEL which inturn generates
beacon alert even though bslot is inactive.

ath9k fails to disable beacon alert while moving to offchannel
if none of the beacon slot is active. This is causing beacon
transmission on foreign channel. This patch enables swba
based on active bslots.

This issue was reported with two vifs (AP+STA) and triggered
scan in STA vif in unassociated state.

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