media: vimc: stream: format comments as kernel-doc

Format the current existing comments as kernel-doc comments, to be
reused at kernel documention. Add opening marks (/**) and return values.

Signed-off-by: André Almeida <andrealmeid@collabora.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>