PCI/AER: Use threaded IRQ for bottom half

The threaded IRQ is naturally single threaded as desired, so use that to
simplify the AER bottom half handler.  Since the root port structure has
much less to do now, remove the rpc construction helper routine.

Signed-off-by: Keith Busch <keith.busch@intel.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>