dm: usb: eth: Support driver model with USB Ethernet

At present USB Ethernet does not work with CONFIG_DM_ETH. Add driver model
support to this feature, so that it can work alongside other Ethernet
devices with driver model.

It was found that quite a bit of code is common in most of the USB Ethernet
drivers. Add this code to the common layer to reduce the amount of duplicate
code needed in USB Ethernet drivers when CONFIG_DM_ETH is used.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Marek Vasut <marex@denx.de>