mac80211: use del_timer_sync for final sta cleanup timer deletion

This is a very old bug, but there's nothing that prevents the
timer from running while the module is being removed when we
only do del_timer() instead of del_timer_sync().

The timer should normally not be running at this point, but
it's not clearly impossible (or we could just remove this.)

Cc: stable@vger.kernel.org
Tested-by: Ben Greear <greearb@candelatech.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>