Fix MQTT async_added_to_hass

[balloob]

Description:

We were not calling async_added_to_hass correctly. It would return a coroutine but with a hackish approach in one and the other was just broken.

Related issue (if applicable): fixes #

Pull request in home-assistant.io with documentation (if applicable): home-assistant/home-assistant.io#<home-assistant.io PR number goes here>

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