mac80211: allow driver to return error from sched_scan_stop

In order to solve races with sched_scan_stop, it is necessary
for the driver to be able to return an error to propagate that
to cfg80211 so it doesn't send an event.

Reviewed-by: Alexander Bondar <alexander.bondar@intel.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>