x86/PCI: MMCONFIG: Check earlier for MMCONFIG region at address zero

We can check for addr being zero earlier and thus avoid the mutex_unlock()
cleanup path.

[bhelgaas: drop warning printk]
Signed-off-by: ethan.zhao <ethan.zhao@oracle.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Acked-by: Yinghai Lu <yinghai@kernel.org>