LinuxKPI: 802.11: fix RSSI (and NF)

As in 4a22cd6c4e5f4fc4a38aa7400742d4005c5ae3de nf and rss should be
signed and not unsigned. Change the types in the header and while
here change a magic number to a define as done elsewhere (value does
not change).

When calculating c_rssi we need to make it relative so subtract nf.
And while here improve the debug output.

This will hopefully fix ifconfig wlanN list scan S:N output which
tools use to chose a BSSID and help net80211 internal calculations.

Sponsored by: The FreeBSD Foundation
MFC after: 3 days