[media] em28xx: set the timestamp type for video and vbi vb2_queues

The em28xx driver obtains the timestamps using function v4l2_get_timestamp(),
which produces a montonic timestamp.
Fixes the warnings appearing in the system log since commit 6aa69f99
"[media] vb2: Add support for non monotonic timestamps"

Signed-off-by: Frank Schäfer <fschaefer.oss@googlemail.com>
Acked-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>