TODO
  ~~~~~~
  Please pick something while reading :)

  - Convert interrupt handler to per-ep-thread-irq
  
    As it turns out some DWC3-commands ~1ms to complete. Currently we spin
    until the command completes which is bad.
  
    Implementation idea:
    - dwc core implements a demultiplexing irq chip for interrupts per
      endpoint. The interrupt numbers are allocated during probe and belong
      to the device. If MSI provides per-endpoint interrupt this dummy
      interrupt chip can be replaced with "real" interrupts.
    - interrupts are requested / allocated on usb_ep_enable() and removed on
      usb_ep_disable(). Worst case are 32 interrupts, the lower limit is two
      for ep0/1.
    - dwc3_send_gadget_ep_cmd() will sleep in wait_for_completion_timeout()
      until the command completes.
    - the interrupt handler is split into the following pieces:
      - primary handler of the device
        goes through every event and calls generic_handle_irq() for event
        it. On return from generic_handle_irq() in acknowledges the event
        counter so interrupt goes away (eventually).
  
      - threaded handler of the device
        none
  
      - primary handler of the EP-interrupt
        reads the event and tries to process it. Everything that requries
        sleeping is handed over to the Thread. The event is saved in an
        per-endpoint data-structure.
        We probably have to pay attention not to process events once we
        handed something to thread so we don't process event X prio Y
        where X > Y.
  
      - threaded handler of the EP-interrupt
        handles the remaining EP work which might sleep such as waiting
        for command completion.
  
    Latency:
     There should be no increase in latency since the interrupt-thread has a
     high priority and will be run before an average task in user land
     (except the user changed priorities).