media: uvcvideo: Allow extra entities

Increase the size of the id, to avoid collisions with entities
implemented by the driver that are not part of the UVC device.

Entities exposed by the UVC device use IDs 0-255, extra entities
implemented by the driver (such as the GPIO entity) use IDs 256 and
up.

Signed-off-by: Ricardo Ribalda <ribalda@chromium.org>
Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>