powerpc/pci: Fix crash in PCI code on ppc64 when matching device nodes

Commit b5d937de has a bug which causes
basically a NULL dereference in the PCI code during boot on ppc64
machines.

fetch_dev_dn() is called when dev->dev.of_node is NULL, so using that
as the starting point for the search makes no sense. It should instead
start from the device node of the PHB.

Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>