media: vivid: add read-only int32 control

This read-only int32 control is used to test read-only controls in
combination with requests. It is set by the driver to the buffer sequence
counter module 256.

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