ACPI: PCI: ignore _PRT function information

_PRT entries don't contain any useful PCI function information (the
function part of the PCI address is supposed to be 0xffff), and we
don't ever look at it, so this patch just removes the reference to
it.

Signed-off-by: Bjorn Helgaas <bjorn.helgaas@hp.com>
Signed-off-by: Len Brown <len.brown@intel.com>