wireless: Do not allow disabled channel in scan request

cfg80211_conn_scan allows disabled channels at scan request.
Hence probe request was seen at the disabled one. This patch
ensures that disabled channel never be added into the scan
request's channel list.

Acked-by: Johannes Berg <johannes@sipsolutions.net>
Signed-off-by: Rajkumar Manoharan <rmanohar@qca.qualcomm.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>