media: v4l2-dev: fix WARN_ON(!vdev->device_caps)

Device nodes of type VFL_TYPE_SUBDEV have a 0 device_caps, so they
trigger this warning. Add a check against VFL_TYPE_SUBDEV.
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Fixes: 3c135050 ("media: v4l2-dev/ioctl: require non-zero device_caps, verify sane querycap results")
Reported-by: Jan Kotas <jank@cadence.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>

drivers/media/v4l2-core/v4l2-dev.c

@@ -859,8 +859,8 @@ int __video_register_device(struct video_device *vdev,
 	/* the v4l2_dev pointer MUST be present */
 	if (WARN_ON(!vdev->v4l2_dev))
 		return -EINVAL;
-	/* the device_caps field MUST be set */
-	if (WARN_ON(!vdev->device_caps))
+	/* the device_caps field MUST be set for all but subdevs */
+	if (WARN_ON(type != VFL_TYPE_SUBDEV && !vdev->device_caps))
 		return -EINVAL;
 
 	/* v4l2_fh support */