wifi: rtw89: debug: add beacon RSSI for debugging

In range test, the RSSI is helpful to check attenuation of cable and align
difference between environments. Since data packets can be transmitted with
different rate and power, the RSSI of all packets can be variant.
Oppositely beacon is transmitted with the same rate and power, so beacon
RSSI will be relatively invariant, and more helpful to diagnose problems.

The output of beacon RSSI in unit of dBm looks like:

  Beacon: 19 (-33 dBm), TF: 0

Signed-off-by: Ping-Ke Shih <pkshih@realtek.com>
Link: https://patch.msgid.link/20240927013512.7106-1-pkshih@realtek.com