rtw88: coex: update BT PTA counter regularly

If BT PTA counter can not update regularly, it will lead coex mechanism
run into some unexpected state.

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://lore.kernel.org/r/20220215004855.4098-4-pkshih@realtek.com