[media] media: v4l2-dev: fix video device index assignment

The side effect of commit 1056e4388b ("v4l2-dev: Fix race condition on
__video_register_device") is the increased number of index value assigned
on video_device registration. Before that commit video_devices were
numbered from 0, after it, the indexes starts from 1, because get_index()
always count the device, which is being registered. Some device drivers
rely on video_device index number for internal purposes, i.e. s5p-mfc
driver stopped working after that patch. This patch restores the old method
of numbering the video_device indexes.

Cc: stable@vger.kernel.org # for v3.12
Signed-off-by: Marek Szyprowski <m.szyprowski@samsung.com>
Acked-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Acked-by: Ricardo Ribalda <ricardo.ribalda@gmail.com>
Signed-off-by: Mauro Carvalho Chehab <m.chehab@samsung.com>

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

@@ -872,8 +872,8 @@ int __video_register_device(struct video_device *vdev, int type, int nr,
 
 	/* Should not happen since we thought this minor was free */
 	WARN_ON(video_device[vdev->minor] != NULL);
-	video_device[vdev->minor] = vdev;
 	vdev->index = get_index(vdev);
+	video_device[vdev->minor] = vdev;
 	mutex_unlock(&videodev_lock);
 
 	if (vdev->ioctl_ops)