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: NJohannes Berg <johannes@sipsolutions.net>
Signed-off-by: NLuciano Coelho <coelho@ti.com>
Signed-off-by: NJohn W. Linville <linville@tuxdriver.com>