wifi: rtw89: record signal strength per RF path

Originally, we show average signal strength. To support TX diversity, this
patch prepares strength per path, then we can decide TX path.

  RSSI: -54 dBm (raw=112, prev=110) [-57, -52]

Signed-off-by: Ping-Ke Shih <pkshih@realtek.com>
Signed-off-by: Kalle Valo <kvalo@kernel.org>
Link: https://lore.kernel.org/r/20220908074140.39776-5-pkshih@realtek.com