make stop signature correct

src/ophyd_async/sim/demo/sim_motor.py

@@ -44,14 +44,16 @@ def __init__(self, prefix: str, name="", instant=True) -> None:
         # Whether set() should complete successfully or not
         self._set_success = True
 
-    def stop(self):
+    def stop(self, success=False):
         """
         Stop the motor if it is moving
         """
         if self._move_task:
             self._move_task.cancel()
             self._move_task = None
 
+        self._set_success = success
+
     def set(self, new_position: float, timeout: Optional[float] = None) -> AsyncStatus:  # noqa: F821
         """
         Asynchronously move the motor to a new position.