ieee1394: change deprecation status of dv1394

Nobody ported ffmpeg from dv1394 to rawiso yet, and there is no
justification to remove dv1394 right now.

Nevertheless, a strong deprecation of this ABI makes a lot of sense,
especially as Kristian H's drivers shape up to be an attractive
alternative to the existing ones.  But we don't have a schedule at the
moment.

Signed-off-by: Stefan Richter <stefanr@s5r6.in-berlin.de>