qtnfmac: implement cfg80211 channel_switch handler

This patch implements cfg80211 channel_switch handler enabling CSA
channel-switch procedure.

Driver performs only basic validation of the requested new channel
and then sends command to firmware. Beacon IEs are not sent since
beacon update is handled by firmware.

Signed-off-by: Igor Mitsyanko <igor.mitsyanko.os@quantenna.com>
Signed-off-by: Sergey Matyukevich <sergey.matyukevich.os@quantenna.com>
Signed-off-by: Avinash Patil <avinashp@quantenna.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>