In the current pciehp, pcied_cleanup() calls destroy_workqueue()
before calling pcie_port_service_unregister(). This causes kernel oops
because flush_workqueue() is called in the pcie_port_service_unregister()
code path after the workqueue was destroyed. So pcied_cleanup() must call
pcie_port_service_unregister() first before calling destroy_workqueue().