media: staging/imx: Missing assignment in imx_media_capture_device_register()

There was supposed to be a "ret = " assignment here, otherwise the
error handling on the next line won't work.

Fixes: 64b5a49d ("[media] media: imx: Add Capture Device Interface")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Reviewed-by: Steve Longerbeam <slongerbeam@gmail.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>