powerpc/pci: Don't configure PCIe settings when PCI_PROBE_ONLY is set

We don't want to configure PCI Express Max Payload Size or
Max Read Request Size on systems that set that flag. The
firmware will have done it for us, and under hypervisors such
as pHyp we don't even see the parent switches and bridges and
thus can make no assumption on what values are safe to use.

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