x86/PCI: MMCONFIG: reject MMCONFIG apertures at address zero

Since all MMCONFIG regions go through pci_mmconfig_add(), we can test the
address once there.  If the caller supplies an address of zero, we never
insert it in the pci_mmcfg_config[] table, so no need to test it elsewhere.

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