The problem is that the code does a check for the granparent of
the Promise chip, if this is a bridge of the right type, we have
a TX4 on our hands, and need to handle that ones "issues".
Now the grandparent check cause subtle bugs in the newbus system,
mainly that pci_get_devid doesn't return an error value.
This patch works around the issue by using BUS_READ_IVAR() instead.
c13beb0171