[media] v4l: Handle buffer timestamp flags correctly

For COPY timestamps, buffer timestamp source flags will traverse the queue
untouched.

Signed-off-by: Sakari Ailus <sakari.ailus@iki.fi>
Acked-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <m.chehab@samsung.com>