mac80211: send null packet on active (psm) reconfiguration

The sta might be in psm against the ap (e.g. because
this was the before a hw restart), so we explicitly
send a null packet in order to make sure it'll
sync against the ap (and get out of psm).

Signed-off-by: Eliad Peller <eliad@wizery.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>