x86, mrst, pci: return 0 for non-present pci bars

Moorestown PCI code has special handling of devices with fixed BARs. In
case of BAR sizing writes, we need to update the fake PCI MMCFG space with real
size decode value.

When a BAR is not present, we need to return 0 instead of ~0. ~0 will be
treated as device error per bugzilla 12006.

Signed-off-by: Jacob Pan <jacob.jun.pan@linux.intel.com>
LKML-Reference: <1273873281-17489-2-git-send-email-jacob.jun.pan@linux.intel.com>
Acked-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Acked-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: H. Peter Anvin <hpa@linux.intel.com>