[urtwn] use/track the last good RSSI for a given node, rather than no RSSI.

Now that we're decap'ing A-MPDU frame, the firmware is only giving us
PHY status information for the whole PPDU, rather than duplicatig it
per frame.

So, we fake it by maintaining the RSSI that we saw in the node struct
and reuse it.

This prevents us from getting some pretty garbage looking default RSSI
values, which shows up as RSSI values of like "3" or "4" when doing
active traffic.

Tested:

* RTL8188EU, STA mode

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