usb: gadget: uvc: uvc_alloc() can be static

The function isn't called from outside of its compilation unit, make it
static.

Signed-off-by: Fengguang Wu <fengguang.wu@intel.com>
Acked-by: Andrzej Pietrasiewicz <andrzej.p@samsung.com>
Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>