net: wifi: Add scan extension for limiting scan results

Add scan extension to allow limiting the scan results to a user defined
maximum number. This parameter is intended to return results with the
best RSSI. It cannot be counted upon to limit the scan times since the
Wi-Fi chips might have to scan all the channels to find APs with the
best RSSI values across all channels.

Signed-off-by: Sachin D Kulkarni <sachin.kulkarni@nordicsemi.no>