ACPI: PCI: simplify buffer management for evaluating _PRT

Previously, acpi_pci_irq_add_prt() did all its own buffer management.
But now that we have ACPI_ALLOCATE_BUFFER, we no longer need to do
that management.  And we don't have to call acpi_get_irq_routing_table()
twice (once to learn the size of the buffer needed, and again to
actually get the table).

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