Fix adding devices on the fly

Devices were not added without persistence enabled and restart of HA.
Node id was added to defaultdict(list) by mistake when checking if list
of defaultdict was empty.

* Fix adding devices in mysensors_callback.
* Change devices to regular dict.

homeassistant/components/sensor/mysensors.py

@@ -7,7 +7,6 @@ For more details about this platform, please refer to the documentation at
 https://home-assistant.io/components/sensor.mysensors/
 """
 import logging
-from collections import defaultdict
 
 from homeassistant.helpers.entity import Entity
 
@@ -72,7 +71,7 @@ def setup_platform(hass, config, add_devices, discovery_info=None):
         v_types = [member for member in gateway.const.SetReq
                    if member.value not in not_v_types]
 
-        devices = defaultdict(list)
+        devices = {}
         gateway.platform_callbacks.append(mysensors.pf_callback_factory(
             s_types, v_types, devices, add_devices, MySensorsSensor))