ieee1394: dv1394: move deprecation message from module init to file open

On many Linux installations, the dv1394 driver will be auto-loaded
whenever an AV/C device (e.g. camcorder or audio device) is plugged in.
An irritating message would then appear in the kernel log.

Defer this message to until a dv1394 character device file is actually
used by a program.  Also include the program name in the message and
update the message slightly.

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