ath9k: Fix beacons for managed mode

If the current opmode is managed, the ATH_OP_BEACONS flag
needs to be set only when there is a primary station interface
and it is associated/active.

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