usb: gadget: ether: convert to new interface of f_subset

teach ethernet code about the new interface of f_subset so
the old one can eventually be removed.

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>