wifi: rtw88: coex: Support chips without a scoreboard

wifi: rtw88: coex: Support chips without a scoreboard

All the chips currently supported have a "scoreboard": the chip keeps
track of certain things related to bluetooth, for example, whether
bluetooth is active. The information can be read from register 0xaa.

RTL8821AU doesn't have this. Implement bluetooth activity detection in
rtw_coex_monitor_bt_enable() based on the bluetooth TX/RX counters.

This is mostly important for RTL8811AU, the version of RTL8821AU without
bluetooth. Without this change, the driver thinks bluetooth is active
and the wifi speeds are low.

Signed-off-by: Bitterblue Smith <rtl8821cerfe2@gmail.com>
Signed-off-by: Ping-Ke Shih <pkshih@realtek.com>
Link: https://patch.msgid.link/5058f23d-2086-42cd-82ad-eef31a348467@gmail.com