ath9k: Fix TSF Adjust usage

TSF adjust is needed only for AP mode when staggered beacons
are used. Since we support only a single interface in IBSS mode,
disable setting the TSF adjust register.

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