[POWERPC] Make sure to of_node_get() the result of pci_device_to_OF_node()

pci_device_to_OF_node() returns the device node attached to a PCI device,
but doesn't actually grab a reference - we need to do it ourselves.

Signed-off-by: Michael Ellerman <michael@ellerman.id.au>
Acked-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Signed-off-by: Paul Mackerras <paulus@samba.org>