wifi: rtw89: 8851b: add set_channel_rf()

Add to set RF registers according to the channel we want to switch. The
callers will be added afterward.

Signed-off-by: Ping-Ke Shih <pkshih@realtek.com>
Signed-off-by: Kalle Valo <kvalo@kernel.org>
Link: https://lore.kernel.org/r/20230424065242.17477-2-pkshih@realtek.com