PCI/MSI: Check that we have a legacy interrupt line before using it

It seems like there are some devices (e.g. the PCIe root port driver) that
may not always have a INTx interrupt.  Check for dev->irq before returning
a legacy interrupt in pci_irq_alloc_vectors to properly handle this case.

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