media: controls: Add VP8 stateless type initialization

The VP8_FRAME_HEADER control type is validated, so it
needs a corresponding initialization to produce default
legal values.

For now, only add the minimum required initialization
to satisfy current validation.

Signed-off-by: Ezequiel Garcia <ezequiel@collabora.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>