mac80211: Notify the driver only when the beacon interval changes

Currently, the driver is unconditionally notified of beacon
interval. This is a problem in AP mode, because the driver has
to know that the beacon interval has actualy changed to recalculate
TBTT and reset the HW TSF. Fix this to make mac80211 notify the driver
only when the beacon interval has been reconfigured to a new value.

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