frontend: fix disconnect when autodetection fails

We don't know the protocol version so we can't actually disconnect,
just skip it.

stcgal/frontend.py

@@ -181,6 +181,7 @@ class StcGal:
         except (StcFramingException, StcProtocolException) as ex:
             sys.stdout.flush()
             print("Protocol error: %s" % ex, file=sys.stderr)
+            if not isinstance(self.protocol, StcAutoProtocol):
                 self.protocol.disconnect()
             return 1
         except serial.SerialException as ex: