media: imx: imx7-media-csi: Create immutable link to capture device

The i.MX7 camera pipeline is simple, with the CSI connected directly to
the capture device without any other option. There's thus no need to
allow link configurable by userspace. Make it immutable.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Rui Miguel Silva <rmfrfs@gmail.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>