cfg80211: fix interface down/disconnect state handling

When the interface goes down, there's no need to call
cfg80211_mlme_down() after __cfg80211_disconnect() as
the latter will call the former (if appropriate.)

Also, in __cfg80211_disconnect(), if the cfg80211 SME
isn't used, __cfg80211_disconnected() may still need
to be called (depending on the current state) so that
the SME state gets cleared.

Signed-off-by: Johannes Berg <johannes.berg@intel.com>