Ensure MQTT subscriptions can be made when the broker is disconnected

[jbouwh]

Proposed change

In case the MQTT entry is enabled, but the broker is not enabled, it should be possible to subscribe. When the broker is connected again the subscriptions will be performed.
This PR fixes a regression caused by removing the client.connected check during a async_subscribe call.

Type of change

• Dependency upgrade
• Bugfix (non-breaking change which fixes an issue)
• New integration (thank you!)
• New feature (which adds functionality to an existing integration)
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• Breaking change (fix/feature causing existing functionality to break)
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Additional information

• This PR fixes or closes issue: fixes #
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[home-assistant]

Hey there @emontnemery, @bdraco, mind taking a look at this pull request as it has been labeled with an integration (mqtt) you are listed as a code owner for? Thanks!

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[jbouwh]

Thanks!