media: v4l2-ctrl: Move compound control initialization

Rework std_init adding an explicit initialization for
compound controls.

While here, make sure the control is initialized to zero,
before providing default values for all its fields.

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