Quirks v2 entities missing unit when setting device class

[GitforZhangXL]

Use the following code to create a sensor entity, but device_class does not automatically include the sensor unit. Does the code need to be modified??

zha/zha/application/platforms/sensor/__init__.py

Line 212 in 57cc44b

if entity_metadata.device_class is None and entity_metadata.unit is not None:

[dmulcahey]

fixed by #219