x86/PCI: MMCONFIG: step through MCFG table, not pci_mmcfg_config[]

Step through the ACPI MCFG table, not pci_mmcfg_config[].  No functional
change, but simplifies future patches that encapsulate pci_mmcfg_config[].

Reviewed-by: Yinghai Lu <yinghai@kernel.org>
Signed-off-by: Bjorn Helgaas <bjorn.helgaas@hp.com>
Signed-off-by: Jesse Barnes <jbarnes@virtuousgeek.org>