x86/PCI: MMCONFIG: remove typeof so we can use a list

This replaces "typeof(pci_mmcfg_config[0])" with the actual type because
I plan to convert pci_mmcfg_config to a list, and then "pci_mmcfg_config[0]"
won't mean anything.

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