demo sample temp: allow stopping

Change-Id: I74ddb8d8c7919e0123431de3833d7f7a4a3c22bd

frappy_demo/modules.py

@@ -248,6 +248,9 @@ class SampleTemp(Drivable):
         # note: setting self.target to the new value is done after this....
         # note: we may also return the read-back value from the hw here
 
+    def stop(self):
+        self.target = self.value
+
     def _thread(self):
         loopdelay = 1
         while True: