[media] Fix vivi regression

This patch fixes a regression introduced by commit
5126f259:

[media] v4l2-dev: add flag to have the core lock all file operations

I forgot to add the locks to the vivi read operation.

Regards,

	Hans

Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>