[IA64] SGI Altix : fix bug in sn_io_late_init()

When initializing pci_controller->node to point to the closest node we need
to take into consideration that a PIC PCI Bridge ASIC can be connected to a
headless/memless node just like the TIOCP and TIOCE Bridge ASICs

Signed-off-by: Mike Habeck <habeck@sgi.com>
Signed-off-by: Tony Luck <tony.luck@intel.com>