x86/pci: Return pci_mmconfig_add() failure early

If pci_mmconfig_alloc() fails, return the failure early so it's obvious
that the failure is the exception, and the success is the normal case.  No
functional change intended.

Link: https://lore.kernel.org/r/20231121183643.249006-9-helgaas@kernel.org


Tested-by: Tomasz Pala <gotar@polanet.pl>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>