media: vivid: add req_validate error injection

Add a new vivid button control to inject an error into the
req_validate request callback.

This will help testing with v4l2-compliance.

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