cfg80211: fix error path in cfg80211_wext_siwscan

If there's an invalid channel or SSID, the code leaks
the scan request. Always free the scan request, unless
it was successfully given to the driver.

Reported-by: Dan Carpenter <error27@gmail.com>
Signed-off-by: Johannes Berg <johannes@sipsolutions.net>
Acked-by: Dan Carpenter <error27@gmail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>