ieee1394: sbp2: don't assume zero model_id or firmware_revision if there is none

This makes sbp2 behave more like firewire-sbp2 which reports 0xff000000
as immediate value if there are no unit directory entries for model_id
or firmware_revision.

It does not reduce matches with the currently existing quirks table; the
only zero entry there is for a device which actually does have a zero
model_id.  It only changes how model_id and firmware_revision are logged
if they are missing.

Other functionally unrelated changes:  The model_id member of quirks
list entries is renamed to model;  the value (but not the effect) of
SBP2_ROM_VALUE_WILDCARD is changed.  Now this part of the source is
identical with firewire-sbp2 for easier maintenance.

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