mac80211: Update beacon's timing and DTIM count on every beacon

Beacon's timestamp, device system time associated with this beacon and
DTIM count parameters are not updated in the associated vif context
if the latest beacon's content is identical to the previously received.
It make sense to update these changing parameters on every beacon so the
driver can get most updated values. This may be necessary, for example,
to avoid either beacons' drift effect or device time stamp overrun.
IMPORTANT: Three sync_* parameters - sync_ts, sync_device_ts and
sync_dtim_count would possibly be out of sync by the time the driver will
use them. The synchronized view is currently guaranteed only in certain
callbacks.

Signed-off-by: Alexander Bondar <alexander.bondar@intel.com>
Signed-off-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>