USB: gadget: Add DEVTYPE support for Ethernet functions

The problem with Ethernet based networking devices is to clearly
identify what's their usage. Special interfaces like bridges, WiFi,
Bluetooth, WiMAX or WWAN are already using DEVTYPE identification.

This patch marks the Ethernet functions of gadgets from the device
type "gadget". Automatic setup of these interfaces can now happen
from userspace without the need of hardcoding the network interface
name.

Signed-off-by: Marcel Holtmann <marcel@holtmann.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>