wifi: rtw89: rfk: concentrate parameter control while set_channel()

For future support on multiple channels, there will be settings of
multiple sub-entities that we need to control. We don't want such
settings to be scattered all over the place. So, we centrally manage
controls of rtw89_phy_idx for RFK in set_channel().

Signed-off-by: Zong-Zhe Yang <kevin_yang@realtek.com>
Signed-off-by: Ping-Ke Shih <pkshih@realtek.com>
Signed-off-by: Kalle Valo <kvalo@kernel.org>
Link: https://lore.kernel.org/r/20220809104952.61355-7-pkshih@realtek.com