rsu: fix RSSI reporting, partially revert r288414.

• Append RCR_APP_PHYSTS bit after firmware loading - otherwise

firmware will reset the register and this modification will be lost.
(without it Rx PHY descriptor section will contain garbage).

• Check if R92S_RXDW0_PHYST bit is set (like it is done in rtwn(4)) -

even if infosz is non-zero the section may not contain anything useful.

• In case, if descriptor is absent (A-MPDU?) use last calibrated RSSI

(rtwn(4) uses RSSI from the previous (sub)frame; probably, this
approach should be used here too).

Tested with Asus USB-N10, STA mode.