usb: dwc3: gadget: Move null pinter check to proper place

usb: dwc3: gadget: Move null pinter check to proper place

When dwc3_gadget_ep_cleanup_completed_requests() called to
dwc3_gadget_giveback() where the dwc3 lock is released, other thread is
able to execute. In this situation, usb_ep_disable() gets the chance to
clear endpoint descriptor pointer which leds to the null pointer
dereference problem. So needs to move the null pointer check to a proper
place.

Example call stack:

Thread#1:
dwc3_thread_interrupt()
  spin_lock
  -> dwc3_process_event_buf()
   -> dwc3_process_event_entry()
    -> dwc3_endpoint_interrupt()
     -> dwc3_gadget_endpoint_trbs_complete()
      -> dwc3_gadget_ep_cleanup_completed_requests()
       ...
       -> dwc3_giveback()
          spin_unlock
          Thread#2 executes

Thread#2:
configfs_composite_disconnect()
  -> __composite_disconnect()
   -> ffs_func_disable()
    -> ffs_func_set_alt()
     -> ffs_func_eps_disable()
      -> usb_ep_disable()
         wait for dwc3 spin_lock
         Thread#1 released lock
         clear endpoint.desc

Fixes: 26288448120b ("usb: dwc3: gadget: Fix null pointer exception")
Cc: stable <stable@kernel.org>
Signed-off-by: Albert Wang <albertccwang@google.com>
Link: https://lore.kernel.org/r/20220518061315.3359198-1-albertccwang@google.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>