modify the rssi logic a bit to actually return a useful rssi.

The fullmac firmware doesn't seem to populate a useful rssi indicator
in the RX descriptor, so if one plotted said values, they'd basically
look like garbage.

The reference driver implements a "get current rssi" firmware command
which I guess is really meant for station operation only (as hostap
operation would need rssi per station, not a single firmware read.)

So:

• populate sc_currssi during each calibration run;
• use this in the RX path instead of trying to reconstruct the RSSI value and passing it around as a pointer;
• do up a quick hack to map the rssi hardware value to some useful signal level;
• the survey results provide an RSSI value between 0..100, so just do another quick hack to map it into some usefulish signal level;
• supply a faked noise floor - I haven't yet found how to pull it out of the firmware.

The scan results and the station RSSI information is now more useful
for indicating signal strength / distance.