Use right equipment identifier in DSMR setup

[dupondje]

Breaking change

Proposed change

The BELGIUM_EQUIPMENT_IDENTIFIER should be used as equipment identifier for 5B. As otherwise this will cause an issue when configuring the DSMR integration for 5B.

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

Hey there @RobBie1221, @frenck, mind taking a look at this pull request as it has been labeled with an integration (dsmr) you are listed as a code owner for? Thanks!

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[joostlek]

Please fix the code coverage in the config flow

[home-assistant]

Please take a look at the requested changes, and use the Ready for review button when you are done, thanks 👍

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