iwm: Fix S:N reporting in ifconfig(8)
While there, rename ieee80211_rx_stats fields nf and rssi to c_nf and
c_rssi, following FreeBSD and other field names of the struct.
With this commit, S:N is properly reported:
> ifconfig wlan0 scan
SSID/MESH ID BSSID CHAN RATE S:N INT CAPS
The Bloom R... 00:e0:4c:b7:65:8c 1 54M -33:-96 100 EP WPA RSN HTCAP WME
0x4e68c3a02... 50:0f:f5:c5:cd:d9 1 54M -82:-96 100 EP HTCAP MESHCONF RSN WME
Genetica ta... b0:be:76:df:af:a6 1 54M -83:-96 100 EPS RSN HTCAP WME ATH WPS
The Bloom R... 00:e0:4c:b7:6a:b0 6 54M -57:-96 100 EP WPA RSN HTCAP WME
...
Fixes: https://bugs.dragonflybsd.org/issues/3366 (#3366)