usb: gadget: uvc: remove an impossible condition

"num" is a u32 so "(num > 0xFFFFFFFF)" is never true.  Also the range
is already checked in kstrtou32().

Acked-by: Andrzej Pietrasiewicz <andrzej.p@samsung.com>
Acked-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>