PCI: Call pci_intx() when using legacy interrupts in pci_alloc_irq_vectors()

ahci currently insists on an explicit call to pci_intx() before falling
back from MSI or MSI-X to legacy IRQs.  As pci_intx() is a no-op if the
command register already contains the right value it seems safe and useful
to add this call to pci_alloc_irq_vectors() so that ahci can just use
pci_alloc_irq_vectors().

Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>