x86/PCI: Constify pci_mmcfg_probes[] array

The pci_mmcfg_probes[] array is only ever read, therefore make it const.

Signed-off-by: Mathias Krause <minipli@googlemail.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Acked-by: Ingo Molnar <mingo@kernel.org>