firewire: log GUID of new devices

This should help to interpret user reports.  E.g. one can look up the
vendor OUI (first three bytes of the GUID) and thus tell what is what.

Also simplifies the math in the GUID sysfs attribute.

Signed-off-by: Stefan Richter <stefanr@s5r6.in-berlin.de>
Signed-off-by: Jarod Wilson <jwilson@redhat.com>