iwm: Fix S:N reporting in ifconfig(8)

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)