PCI/portdrv: Use pci_irq_alloc_vectors()

Use pci_irq_alloc_vectors() and greatly simplify the code by managing the
vector number for the subservices directly.

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