USB: pl2303: New vendor and product id for the prolific driver

I recently bought a PC interface for the Cressi Edy dive computer
(www.cressi.it) and discovered that it uses the pl2303 chip, albeit
with ad-hoc vendor and product ids (04b8, 0521 respectively). Being in
the process of writing a linux software for such device (cressi only
provides a windows software), I patched the pl2303 linux driver to
have the interface recognized. I am submitting you the patch (very
basic) for inclusion in next kernel version.

From: Gianpaolo Cugola <gianpaoloc@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>