mac80211: reduce number of __ieee80211_scan_completed calls

Use goto instruction to call __ieee80211_scan_completed only ones in
ieee80211_scan_work. This is prepare for the next patch.

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