mac80211: fix for mesh sync to indicate TBTT adjustment

Currently the mesh sync code checks, whether peers indicate TBTT adjustment,
but it never sets the corresponding flag itself.

By setting ifmsh->tbtt_adjusting to true, it will set the corresponding field
in the mesh configuration IE of own beacons.
This indication will be set in the current beacon. The TBTT adjustment will be
performed afterwards, affecting the next beacon. Thus, the first beacon with
stable TBTT will not indicate adjustment anymore and peers will continue
tracking the new offset.

Signed-off-by: Marco Porsch <marco.porsch@etit.tu-chemnitz.de>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>