ath9k: Further fix for mesh beaconing

This fix ensure the timers to be set at beacon interval boundaries.
Without this change timers can be set improperly resulting in the absence of beacons.

Signed-off-by: Fabrice Deyber <fabricedeyber@agilemesh.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>