[rsu] convert rsu to use the ieee80211_rx_stats struct to pass up RSSI, PHY and rate information.

I don't yet know which RX descriptor bits map to shortgi, long-gi,
short-preamble, long-preamble, STBC, LDPC, HT40, etc - so I can't
easily add those just yet.

There's apparently no per-frame RX RSSI information exposed so we
also just use the results from the previous calibration task.

This also tidies up how the per-mbuf RSSI is pushed into the frame -
now that it's attached to the mbuf via rx_stats, we don't have to
do any silly hijinx to get it out of the frame processing path.

Tested:

• RTL8712, 1x1 cut 3, STA mode