PCI: Fix IRQ swizzling for ARI-enabled devices

For many purposes, including interrupt-swizzling, devices with ARI
enabled behave as if they have one device (number 0) and 256 functions.
This probably hasn't bitten us in practice because all ARI devices I've
seen are also IOV devices, and IOV devices are required to use MSI.
This isn't guaranteed, and there are legitimate reasons to use ARI
without IOV, and hence potentially use pin-based interrupts.

Signed-off-by: Matthew Wilcox <willy@linux.intel.com>
Signed-off-by: Jesse Barnes <jbarnes@virtuousgeek.org>