mac80211: abort scan_work immediately when the device goes down

As long as no delay is required b/w channel change, scan work
is proceeding without scheduling a new work. In such case, we
can not abort scan work when the card was unplugged. This patch
completes the scanning immediately whenever the device goes down.

Reviewed-by: Johannes Berg <johannes@sipsolutions.net>
Signed-off-by: Rajkumar Manoharan <rmanoharan@atheros.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>