PCI: iproc: Configure PCIe MPS settings

Make sure PCIe MPS settings are valid when we enumerate a new hierarchy.

[bhelgaas: changelog]
Signed-off-by: Jon Mason <jon.mason@broadcom.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Acked-by: Ray Jui <ray.jui@broadcom.com>