PCI: Add pci_msix_desc_addr() helper

Add a pci_msix_desc_addr() helper to factor out the calculation of the base
address for a given MSI-X vector.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Reviewed-by: Alexander Gordeev <agordeev@redhat.com>