iwlagn: clean up some exit code

There's no point in running through iwl_down()
when we never registered with mac80211, as it
just cleans up internal structures that were
never initialised in this case. Therefore we
can also remove the special handling for this
case from __iwl_down().

Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Wey-Yi Guy <wey-yi.w.guy@intel.com>