mac80211: acquire sta_lock for station suspend/resume

To avoid concurrent manipulations of the sta list (which shouldn't
be possible at this point, but anyway) we need to hold the sta_lock
around iterating the list.

At the same time, we do not need to iterate the list at all if
the driver doesn't want to be notified.

Signed-off-by: Johannes Berg <johannes@sipsolutions.net>
Acked-by: Bob Copeland <me@bobcopeland.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>