rtw88: coex: add power off setting

Clear WL/BT on/off bit in scoreboard register that is used to exchange
WL/BT status. Since the status is preserved after warm reboot, we must
clear it when WL is going to down.

Signed-off-by: Ping-Ke Shih <pkshih@realtek.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>
Link: https://lore.kernel.org/r/20210219054140.7835-1-pkshih@realtek.com