usb: gadget: uvc: Fix return value in case of error

If this memory allocation fail, we will return 0, which means success.
Return -ENOMEM instead.

Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Signed-off-by: Felipe Balbi <felipe.balbi@linux.intel.com>