media: sun6i-csi: Register the media device after creation

There is no particular need to register the media device in the
subdev notify complete callback.

Register it in the v4l2 code instead where it's more in-context.

Signed-off-by: Paul Kocialkowski <paul.kocialkowski@bootlin.com>
Acked-by: Jernej Skrabec <jernej.skrabec@gmail.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab@kernel.org>