media: imx: capture: Register the video device after completing init

When the video device is registered, it can get used by userspace
immediately. Its initialization must thus be fully complete. Ensure this
by registering the video device after all initialization steps.

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>