powerpc/pci: clean up direct access to sysdata by iseries platform

We shouldn't directly access sysdata to get the device node.  We should
be calling pci_device_to_OF_node().

Signed-off-by: Kumar Gala <galak@kernel.crashing.org>
Tested-by: Stephen Rothwell <sfr@canb.auug.org.au>
Acked-by: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>