media: uvcvideo: Constify descriptor buffers

There is no need to modify the content of UVC descriptor buffers during
parsing. Make all the corresponding pointers const to avoid unintended
modifications.

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