V4L/DVB (10544): v4l2-common: add comments warning that about the sort order

Control arrays as are used with v4l2_ctrl_next must be sorted from
low to high. Add a comment at the top of all such arrays to warn
about this.

Signed-off-by: Hans Verkuil <hverkuil@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>
diff --git a/drivers/media/video/cx18/cx18-controls.c b/drivers/media/video/cx18/cx18-controls.c
index 6af4d5c..10a4e07 100644
--- a/drivers/media/video/cx18/cx18-controls.c
+++ b/drivers/media/video/cx18/cx18-controls.c
@@ -30,6 +30,7 @@
 #include "cx18-mailbox.h"
 #include "cx18-controls.h"
 
+/* Must be sorted from low to high control ID! */
 static const u32 user_ctrls[] = {
 	V4L2_CID_USER_CLASS,
 	V4L2_CID_BRIGHTNESS,