media: uvcvideo: Only create input devs if hw supports it

Examine the stream headers to figure out if the device has a button and
can be used as an input.

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