dm: usb: Avoid using USB ethernet with CONFIG_DM_USB and no DM_ETH

If driver model is used for Ethernet then USB Ethernet does not build. This
can be made to work with driver model is used for USB also. Add #ifdef logic
to make this clear when building.

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