drivers: usb: gadget: ether: use net device priv to pass usb ether priv

Use net device priv to pass usb ether priv and use it in
net device ops callback.

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