x86/PCI: MMCONFIG: rename pci_mmcfg_region structure members

This only renames the struct pci_mmcfg_region members; no functional change.

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