PCI: Check all ACS features for multifunction downstream ports

The multifunction ACS rules do not apply to downstream ports.  Those
should be tested regardless of whether they are single function or
multifunction.  The PCIe spec also fully specifies which PCIe types
are subject to the multifunction rules and excludes event collectors
and PCIe-to-PCI bridges entirely.  Document each rule to the section
of the PCIe spec and provide overall documentation of the function.

Signed-off-by: Alex Williamson <alex.williamson@redhat.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Acked-by: Donald Dutile <ddutile@redhat.com>