rtw88: 8821c: fix debugfs rssi value

RSSI value per frame is reported to mac80211 but not maintained in
our own statistics, add it back to help us debug.

Signed-off-by: Po-Hao Huang <phhuang@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/20220407095858.46807-7-pkshih@realtek.com