dm: usb: fix USB Ethernet without CONFIG_DM_ETH regression

dm: usb: fix USB Ethernet without CONFIG_DM_ETH regression

The following commit enforces CONFIG_DM_ETH for USB Ethernet which
breaks any board using CONFIG_USB_HOST_ETHER without CONFIG_DM_ETH
which this patch fixes.

commit 69559093f6173dcfcb041df0995063bdbd07d49b
dm: usb: Avoid using USB ethernet with CONFIG_DM_USB and no DM_ETH

Tested on Colibri T20/T30 as well as Apalis T30 with
CONFIG_USB_HOST_ETHER and CONFIG_USB_ETHER_ASIX enabled and a LevelOne
USB-0301 ASIX AX88772 dongle.

Signed-off-by: Marcel Ziswiler <marcel.ziswiler@toradex.com>
Acked-by: Simon Glass <sjg@chromium.org>