mac80211: internally clear failed scans properly

When the IBSS code wants to scan, but that fails, we can
get stuck in a situation where you can never scan again.
Fix this by properly notifying ourselves when the scan
request has failed.

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