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

The side effect of commit 1056e438 ("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>