mac80211: add stations after AP start on reconfig

When performing a HW restart for an AP mode interface, add stations back
only after the AP is beaconing. This mimics the normal flow of STA
addition on AP.

Some devices (wlcore) do not support adding stations before beaconing,
so this has the added benefit of making recovery work for them.

Signed-off-by: Arik Nemtsov <arik@wizery.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>