[media] V4L: Merge struct v4l2_async_subdev_list with struct v4l2_subdev

By integrating the v4l2-async API internals a bit more with
the core overall the v4l2-async code becomes a bit simpler
and easier to follow.
Acked-and-tested-by: Lad, Prabhakar <prabhakar.csengg@gmail.com>

Signed-off-by: Sylwester Nawrocki <s.nawrocki@samsung.com>
Signed-off-by: Kyungmin Park <kyungmin.park@samsung.com>
Acked-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Acked-by: Hans Verkuil <hans.verkuil@cisco.com>
Acked-by: Guennadi Liakhovetski <g.liakhovetski@gmx.de>
Signed-off-by: Mauro Carvalho Chehab <m.chehab@samsung.com>