media: staging/imx: Fix uninitialized variable warning

The ret variable can be returned uninitialized in the
imx_media_create_pad_vdev_lists() function is imxmd->num_vdevs is zero.
Fix it.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Hans Verkuil <hansverk@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>