rtw89: make rfk helpers common across chips

These rfk helpers are also useful for the chip which is under planning.
So, move them to common code to avoid duplicate stuff in the future.

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/20220211075953.40421-2-pkshih@realtek.com