usb: gadget: u_ether: construct with default values and add setters/getters

Add an interface to create a struct netdev_dev filled with default values, an
interface which makes it an interface to fill the struct with useful values and
an interface to read the values set.

The patch also adds an interface to register the net device associated with an
ethernet-over-usb link.

Signed-off-by: Andrzej Pietrasiewicz <andrzej.p@samsung.com>
Signed-off-by: Kyungmin Park <kyungmin.park@samsung.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>