cfg80211: allow scanning on specified frequencies when using wext-compatibility

Handles the case when SIOCSIWSCAN specified iw_scan_req.num_channels and
iw_scan_req.channels[].

Signed-off-by: Holger Schurig <hs4233@mail.mn-solutions.de>
Signed-off-by: John W. Linville <linville@tuxdriver.com>