Change rssi passed to ifconfig(8) to a signed value to fix printing signal

Change rssi passed to ifconfig(8) to a signed value to fix printing signal
strengths on 802.11 interfaces.  ok stsp@

Power on wireless interfaces is usually scaled in dBm. rssi (received signal
strength) is expected to be a -ve dBm values (i.e. [much] <1mW). Some (though
not all!) drivers store this as negative values, so it needs to be passed to
ifconfig that way for printing, not cast to an unsigned value. Valid range is
something like -40 to -90dBm, so the range available with a signed char is
reasonable whether it's stored as a +ve or -ve number.