Unify RSSI check

Unify RSSI check

Two spots in ifconfig print the signal strength, one for the "ieee80211: "
line and one for each SSID in scan output.

Only the former checks nr_rssi, which is not needed as nr_max_rssi alone
indicates whether the driver reports signal strength in percentage or dBm.

Zap the nr_rssi check to simplify;  "worst case" we display 0% or 0dBm
instead of nothing.

Feedback OK stsp