rtw88: coex: add the mechanism for RF4CE

Add the related coexistence mechanism about RF4CE and WLAN connected
flag to decide control packet is whether higher or normal priority.

Signed-off-by: Ching-Te Ku <ku920601@realtek.com>
Signed-off-by: Ping-Ke Shih <pkshih@realtek.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>
Link: https://lore.kernel.org/r/20201112031430.4846-6-pkshih@realtek.com