frappy_psi.nanov: minor fixes

Change-Id: I0c6f1f09358155f49d4dedfe0c254dbe6b514b36

frappy_psi/nanov.py

@@ -45,18 +45,18 @@ class NanovIO(StringIO):
             channel = self._channels[idx]
             reply = float(self.communicate('FETCH?'))
             if abs(reply) > 1000:
-                print('BAD', reply)
+                self.log.warning('bad reply: %s', reply)
                 return
             channel.value = float(reply)
         except IndexError:
             idx = -1
         now = time.time()
         if now < self._last_change + 5 or (idx >= 0 and len(self._channels) == 1):
-            return idx
+            return
         self._last_change = now
         self._channel_index = idx = (idx + 1) % len(self._channels)
         self.channel = self._channels[idx].channel
-        result = self.communicate(';SENS:CHAN %i;:SENS:CHAN?' % self.channel)
+        self.communicate(';SENS:CHAN %i;:SENS:CHAN?' % self.channel)
 
 
 class Volt(HasIO, Readable):