ath9k: Fix improper beacon slot selection in IBSS

Request a re-configuration of Beacon related timers
on the receipt of the first Beacon frame has to be set only
for station mode. Setting beacon sync for IBSS is causing
wrong beacon slot selection on beacon generation.

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