rtl8xxxu: Fix wifi low signal strength issue of RTL8723BU

rtl8xxxu: Fix wifi low signal strength issue of RTL8723BU

The WiFi tx power of RTL8723BU is extremely low after booting. So
the WiFi scan gives very limited AP list and it always fails to
connect to the selected AP. This module only supports 1x1 antenna
and the antenna is switched to bluetooth due to some incorrect
register settings.

Compare with the vendor driver https://github.com/lwfinger/rtl8723bu,
we realized that the 8723bu's enable_rf() does the same thing as
rtw_btcoex_HAL_Initialize() in vendor driver. And it by default
sets the antenna path to BTC_ANT_PATH_BT which we verified it's
the cause of the wifi weak tx power. The vendor driver will set
the antenna path to BTC_ANT_PATH_PTA in the consequent btcoexist
mechanism, by the function halbtc8723b1ant_PsTdma.

This commit hand over the antenna control to PTA(Packet Traffic
Arbitration), which compares the weight of bluetooth/wifi traffic
then determine whether to continue current wifi traffic or not.
After PTA take control, The wifi signal will be back to normal and
the bluetooth scan can also work at the same time. However, the
btcoexist still needs to be handled under different circumstances.
If there's a BT connection established, the wifi still fails to
connect until BT disconnected.

Signed-off-by: Chris Chiu <chiu@endlessm.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>