rtw89: coex: set EN bit to PLT register

B_AX_PLT_EN is to enable polluted mechanism. If it is enabled and
gnt_bt = 1 while wlan TX, B_AX_BT_PLT_PKT_CNT counter will increase,
but TX counter to BB will not. Without this bit BTCoex mechanism might
have some problems.

Signed-off-by: Ping-Ke Shih <pkshih@realtek.com>
Signed-off-by: Kalle Valo <kvalo@kernel.org>
Link: https://lore.kernel.org/r/20220208082751.43553-1-pkshih@realtek.com