wcn36xx: Implement firmware assisted scan

Using the software based channel scan mechanism from mac80211 keeps us
offline for 10-15 second, we should instead issue a start_scan/end_scan
on each channel reducing this time.

Signed-off-by: Bjorn Andersson <bjorn.andersson@linaro.org>
Signed-off-by: Kalle Valo <kvalo@qca.qualcomm.com>