rtw88: 8821c: Correct CCK RSSI

Incorrect CCK RSSI may cause periodically scan from upper layer.

8821c phy status does NOT has actual value of CCK power. It provides only
lna and vga index. Driver have to use these indexes to calculate
actual RSSI.

Signed-off-by: Guo-Feng Fan <vincent_fann@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/20210202055012.8296-3-pkshih@realtek.com