x86/PCI: MMCONFIG: simplify tests for empty pci_mmcfg_config table

We never set pci_mmcfg_config unless we increment pci_mmcfg_config_num,
so there's no need to test both pci_mmcfg_config_num and 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>