drivers: usb: gadget: ether: adopt to usb driver model

Convert usb ether gadget to adopt usb driver model

Signed-off-by: Mugunthan V N <mugunthanvnm@ti.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
Acked-by: Joe Hershberger <joe.hershberger@ni.com>