mac80211: mesh: flush stations before beacons are stopped

Some drivers (e.g. wl18xx) expect that the last stage in the
de-initialization process will be stopping the beacons, similar to AP flow.
Update ieee80211_stop_mesh() flow accordingly.
As peers can be removed dynamically, this would not impact other drivers.

Tested also on Ralink RT3572 chipset.

Signed-off-by: Maital Hahn <maitalm@ti.com>
Signed-off-by: Yaniv Machani <yanivma@ti.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>