mac80211: flush stations before stop beaconing

When AP interface is going down, the stations
are flushed (in ieee80211_do_stop()) only after
the beaconing was stopped.

However, drivers might rely on stations being
removed before the beaconing was stopped, in
order to clean up properly.

Fix it by flushing the stations on ap stop.

(we already do the same for other interface
types, e.g. in ieee80211_set_disassoc())

Signed-off-by: Eliad Peller <eliad@wizery.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>