usb: gadget: f_uvc: convert f_uvc to new function interface

Use the new function registration interface. It is required
in order to integrate configfs support.

Signed-off-by: Andrzej Pietrasiewicz <andrzej.p@samsung.com>
Tested-by: Michael Grzeschik <m.grzeschik@pengutronix.de>
[Updated copyright years]
Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Acked-by: Andrzej Pietrasiewicz <andrzej.p@samsung.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>