If you just have one subsystem ID, DMI might be a good idea. I suggested not to use the IDE controller for the second subsystem ID, as it varies with AHCI/RAID/whatsoever modes, but this board is so old, it has nothing of them. I suggest to use the IDE PCI ID with subsystem as second device and drop the DMI.
Yes in fact the only settings you can change in the bios are related to UDMA modes and such. And btw, I was wrong on iRC - all USB controllers can be disabled.
Attached patch updated as suggested & tested.
Regards, David Borg