media: docs: v4l2-controls: rearrange control initialization sequence

The code snippet showing how to add controls to the driver’s top-level
struct is present twice, but only the second time it is split in the V4L2
and subdev cases. Consolidate everything at the beginning.

Also remove the "Where foo->bar is of type struct baz" sentences, this
obvious from the code.

Signed-off-by: Luca Ceresoli <luca@lucaceresoli.net>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>