usb: gadget: cdc2: convert to new interface of f_ecm

f_ecm has been converted to new configfs infrastructure,
fixing cdc2 gadget driver.

[ balbi@ti.com : fixed a bunch of errors when adding ECM function ]

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>