wifi: rtw89: mac: add coexistence helpers {cfg/get}_plt

When hardware grant BT initially but transition to grant WiFi, the PLT
(polluted) bit is set to assist coexistence mechanism to debug if
grant signal is expected.

Signed-off-by: Ping-Ke Shih <pkshih@realtek.com>
Signed-off-by: Kalle Valo <kvalo@kernel.org>
Link: https://msgid.link/20240229074514.219276-2-pkshih@realtek.com