MySensorsMulti attribute values are not properly updated in variable expressions & rules

[mwittig]

See also:

• https://github.com/DheerajKhajuria/pimatic-mysensors/issues/46
• https://forum.pimatic.org/topic/945/pimatic-dewpoint-device-doesn-t-update/13

[GeeF]

This fixes the issue with a soil moisture sensor I have, that wouldn't properly update if added as a Multi.