mac80211: compete scan to cfg80211 if deferred scan fail to start

We nulify local->scan_req on failure in __ieee80211_start_scan, so
__ieee80211_scan_completed will not call cfg80211_scan_done. Fix that.

Signed-off-by: Stanislaw Gruszka <sgruszka@redhat.com>
Acked-by: Johannes Berg <johannes@sipsolutions.net>
Signed-off-by: John W. Linville <linville@tuxdriver.com>