usb: ether: Fix error handling in usb_ether_init

The code checks the return value from uclass_first_device as well as
that the device exists but it passes on the return value which may be
zero if there are no gadget devices. Just check that a device was
returned and return -ENODEV otherwise.

Also remove the dev variable which is not really used for anything.

Signed-off-by: Michal Suchanek <msuchanek@suse.de>
Reviewed-by: Simon Glass <sjg@chromium.org>