[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>

include/linux/uvcvideo.h

@@ -58,7 +58,8 @@ struct uvc_xu_control_mapping {
 struct uvc_xu_control_query {
 	__u8 unit;
 	__u8 selector;
-	__u8 query;
+	__u8 query;		/* Video Class-Specific Request Code, */
+				/* defined in linux/usb/video.h A.8.  */
 	__u16 size;
 	__u8 __user *data;
 };