[media] uvcvideo: Document the struct uvc_xu_control_query query field

Several developers have reported that the lack of macros for the struct
uvc_xu_control_query query field in uvcvideo.h was confusing and forced
them to look at the driver source code to find out what applications
were supposed to pass in that field.

Add a comment to the header to clarify the expected usage of the query
field.

Reported-by: Paulo Assis <pj.assis@gmail.com>
Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>