PCI irq affinity fix for 4.12

[Christoph Hellwig]

Hi Bjorn,

Steve found a case where the !CONFIG_PCI_MSI stub for pci_alloc_irq_vectors
did the wrong thing when used with the newly converted xhci driver.  While
the PCI code has been wrong since day 1 the fact that a newly converted
drivers triggers it makes it a regression in this cycle.