[media] v4l2-subdev: convert documentation to the right format

struct v4l2_subdev_core_ops has some kernel-doc-nano documentation
using a wrong format, with affects the number of stuff reported at
the DocBook device-drivers.xml.

Properly mark the such comment blocks using the right notation.

Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>
Acked-by: Jonathan Corbet <corbet@lwn.net>