usb: gadget: cdc2: fix conversion to new interface of f_ecm

This fixes commit a38a2750
(usb: gadget: cdc2: convert to new interface of f_ecm)

The invocation of usb_get_function_instance() is in cdc_bind()
and should not be repeated in cdc_do_config().

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>