net: usb: cdc_ether: add Dell DW5580 as a mobile broadband adapter

Since Dell DW5580 is a 3G modem, this patch adds the device as a
mobile broadband adapter

Signed-off-by: Daniele Palmas <dnlplm@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>