powerpc/pci: Clean up direct access to sysdata by powermac platforms

We shouldn't directly access sysdata to get the device node but call
pci_bus_to_OF_node() for this purpose.

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