gateware.usb.usb2.request: stall when handlers step on each other

This commit changes the fallback mechanism to also stall when multiple
request handlers want to manage a setup packet at the same time.

luna/gateware/usb/usb2/control.py

@@ -135,7 +135,7 @@ def elaborate(self, platform):
         if len(self._request_handlers) > 0:
             # Emit a stall when the request is not taken care of by the registered request handlers
             handle_conditions = [handler.handled for handler in self._request_handlers]
-            stall_condition = lambda setup: reduce(operator.and_, (~cond(setup) for cond in handle_conditions))
+            stall_condition = lambda setup: sum((cond(setup) for cond in handle_conditions)) != 1
         else:
             stall_condition = lambda setup: True
         self.add_request_handler(StallOnlyRequestHandler(stall_condition))