usb: gadget: nokia: convert to new interface of f_obex

preparation to use configfs-based approach on g_nokia.ko

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>