media: staging/imx: Move add_video_device into capture_device_register

Move imx_media_add_video_device() into imx_media_capture_device_register().
Also the former has no error conditions to convert to void.

Signed-off-by: Steve Longerbeam <slongerbeam@gmail.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>