media: staging: media: imx: imx7-media-csi: Drop imx_media_add_video_device call

There's no need to call imx_media_add_video_device() anymore, as the
video devices list it manages is only used by the control inheritance
mechanism in the helpers, which this driver doesn't use.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Acked-by: Rui Miguel Silva <rmfrfs@gmail.com>
Tested-by: Alexander Stein <alexander.stein@ew.tq-group.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@kernel.org>