cfg80211: return -ENOENT when stopping sched_scan while not running

If we try to stop a scheduled scan while it is not running, we should
return -ENOENT instead of simply ignoring the command and returning
success.  This is more consistent with other parts of the code.

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