ofono_client: Power off modem during cleanup()

Change-Id: Ic0c509e77ababf385beb8b1d26659ca4d18f9331
diff --git a/src/osmo_gsm_tester/ofono_client.py b/src/osmo_gsm_tester/ofono_client.py
index 70fee25..60f98df 100644
--- a/src/osmo_gsm_tester/ofono_client.py
+++ b/src/osmo_gsm_tester/ofono_client.py
@@ -167,6 +167,7 @@
         self.connected_signals = util.listdict()
 
     def cleanup(self):
+        self.set_powered(False)
         self.unwatch_interfaces()
         for interface_name in list(self.connected_signals.keys()):
             self.remove_signals(interface_name)