Fix field validation for mqtt subentry options in sections

[jbouwh]

Proposed change

Fields in the data entry flow that are in sections should tag errors to their sections. It seems not possible to add the error message label directly to the field in the section.

This PR fixes the cases where an invalid field tag was applied because the field was inside a section.

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)
• Deprecation (breaking change to happen in the future)
• Breaking change (fix/feature causing existing functionality to break)
• Code quality improvements to existing code or addition of tests

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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