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

We shouldn't directly access sysdata to get the pci_controller.  Instead
use pci_bus_to_host() for this purpose.  In the future we might have
sysdata be a device_node to match ppc64 and unify the code between ppc32
& ppc64.

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