x86/PCI: MMCONFIG: count MCFG structures with local variable

Use a local variable, not pci_mmcfg_config_num, to count MCFG entries.
No functional change, but simplifies future changes.

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