usbnet: Set device type for wlan and wwan devices

For usbnet devices with FLAG_WLAN and FLAG_WWAN set the proper device
type so that uevent contains the correct value. This then allows an easy
identification of the actual underlying technology of the Ethernet device.

Signed-off-by: Marcel Holtmann <marcel@holtmann.org>
Signed-off-by: David S. Miller <davem@davemloft.net>