[media] v4l2-ioctl: WARN_ON if querycap didn't fill device_caps

This is easy to forget to do in drivers. While v4l2-compliance will check for it,
not everyone remembers to run it. So warn about it.
Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>

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

@@ -1017,6 +1017,12 @@ static int v4l_querycap(const struct v4l2_ioctl_ops *ops,
 	ret = ops->vidioc_querycap(file, fh, cap);
 
 	cap->capabilities |= V4L2_CAP_EXT_PIX_FORMAT;
+	/*
+	 * Drivers MUST fill in device_caps, so check for this and
+	 * warn if it was forgotten.
+	 */
+	WARN_ON(!(cap->capabilities & V4L2_CAP_DEVICE_CAPS) ||
+		!cap->device_caps);
 	cap->device_caps |= V4L2_CAP_EXT_PIX_FORMAT;
 
 	return ret;