wcn36xx: Add hardware scan offload support

Current hw_scan implementation does not trigger offloaded
hardware scan and seems to only put the device in a kind of
listening mode (beacon/probe-response) for software scan.
Since no probe request are generated by the software, current
scanning method is similar to a passive scan.

This patch introduces support for 'true' hardware offloaded scan.
Hardware scan is configured and started via the start-scan-offload
firmware message. Once scan has been completed a scan indicator
message is received from firmware.

Moreover, this patch includes support for directed probe-request,
allowing connection with hidden APs. It also fixes scan issues with
band-steering AP which are not 'visible' with passive scan (due to
hidden ssid in beacons).

Let's keep the 'legacy' scanning method in case scan-offload is not
supported.

Signed-off-by: Loic Poulain <loic.poulain@linaro.org>
Signed-off-by: Kalle Valo <kvalo@qca.qualcomm.com>