mac80211: fix sw scan locking

The recent scan overhaul broke locking
because now we can jump to code that
attempts to unlock, while we don't have
the mutex held. Fix this by holding the
mutex around all the relevant code.

Reported-by: Ben Greear <greearb@candelatech.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>