iwlwifi: mvm: make UMAC scans use the stopping scan status

UMAC scans now use the general scan status for almost everything, the
only part missing was in the scan complete notifications.  Change it
to use the stopping flags instead of clearing the flags when the stop
comes from above and clean the handler function a bit.

Signed-off-by: Luciano Coelho <luciano.coelho@intel.com>
Reviewed-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>