usb: dwc3: gadget: rework the dequeue on RESET & DISCONNECT

- since a while we are disabling an endpoint and purging every requests on
  RESET and DISCONNECT which leads to a warning since the endpoint was
  disabled twice (once by the UDC, and second time by the gadget). I
  think UDC should nuke all requests because all those requests
  become invalid. It's gadget driver's responsability, though, to disable
  its used endpoints. This is done by merging dwc3_stop_active_transfer()
  and dwc3_gadget_nuke_reqs() into dwc3_remove_requests().

- dwc3_stop_active_transfer() is now no longer called unconditionaly.
  This has the advantage that it is always called to disable an active
  transfer which means if res_trans_idx 0 than something went wrong and
  it is an error condition because we can't clean up the requests.

- Remove the DWC3_EP_WILL_SHUTDOWN which was introduced while
  introducing the command complete part for dequeue. All requests on
  req_queued list should be removed during the dwc3_cleanup_done_reqs()
  callback so there is no reason to go through the list again.
  We consider it an error condition if requests are still on this
  list since we never queue TRB without LST=1 (the last requests has
  always LST=1, there are no requests with LST=0 behind it).

[ balbi@ti.com : reworked commit log a bit, made patch apply ]

Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: Felipe Balbi <balbi@ti.com>