ACPI: PCI: simplify list of _PRT entries

We don't need a struct containing a count and a list_head; a simple
list_head is sufficient.  The list iterators handle empty lists
fine.

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