media: v4l2-ctrl: Initialize _BUTTON and _CTRL_CLASS

These two control types don't really need a default value,
as they are not expected to carry any value.

However, it's slightly clearer to initialize them explicitly
instead of falling back to the switch default.

Signed-off-by: Ezequiel Garcia <ezequiel@collabora.com>
Reviewed-by: Boris Brezillon <boris.brezillon@collabora.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>