drivers: usb: gadget: ether: consolidate global devices to single struct

Consolidate the net device, usb eth device and gadget device
struct to single struct and a single global variable so that the
same can be passed as priv of ethernet driver.

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