usb: gadget: ether: put_usb_function on unbind

Fix bugs introduced in

9c62ce83
usb: gadget: ether: convert to new interface of f_ecm

94b5573e
usb: gadget: ether: convert to new interface of f_eem

8af5232d
usb: gadget: ether: convert to new interface of f_subset

9bd4a10e
usb: gadget: ether: convert to new interface of f_rndis

Acked-by: Michal Nazarewicz <mina86@mina86.com>
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>