athk6l: Fix channel list processing in scan requests

Limit the length of the channel list to WMI_MAX_CHANNELS to avoid
rejection of the request in wmi.c. Since there is not really much
point in using a specific list of more than 32 channels, drop the
channel list if more channels are specified and scan all channels.

Fix cfg80211 scan API use: ar->scan_req must be set only if returning
success from scan() handler. The previous version would result in use
of freed memory and likely kernel panic should the scan request fail
to be sent to the target.

Signed-off-by: Jouni Malinen <jouni@qca.qualcomm.com>
Signed-off-by: Kalle Valo <kvalo@qca.qualcomm.com>