The whole algorithm was wrong.  What we need to do is:

1) Walk each PCI bus above this device on the path to the
   PCI controller nexus, and for each:
      a) If interrupt-map exists, apply it, record IRQ controller node
      b) Else, swivel interrupt number using PCI_SLOT(), use PCI bus
	 parent OBP node as controller node
      c) Walk up to "controller node" until we hit the first PCI bus
	 in this domain, or "controller node" is the PCI controller
	 OBP node
2) If we walked to PCI controller OBP node, we're done.
3) Else, apply PCI controller interrupt-map to interrupt.

There is some stuff that needs to be checked out for ebus and
isa, but the PCI part is good to go.
Signed-off-by: David S. Miller <davem@davemloft.net>