wifi: rtw89: coex: Update BTG control related logic

BTG is a RF system type, it means Wi-Fi 2.4GHz and Bluetooth share RF gain
and antenna. The RF gain must control by Wi-Fi or Bluetooth in single side.
For example, if Bluetooth RX a very strong signal, then Bluetooth will
adjust to a lower gain. And Wi-Fi will also use the same gain to do RX,
then maybe the gain will not enough. This BTG control mechanism can do
some refine to this situation.

Signed-off-by: Ching-Te Ku <ku920601@realtek.com>
Signed-off-by: Ping-Ke Shih <pkshih@realtek.com>
Signed-off-by: Kalle Valo <kvalo@kernel.org>
Link: https://msgid.link/20231218061341.51255-5-pkshih@realtek.com