drivers:adc:ad713x:Bug fix with AD7134 driver

[jansunil]

Bug fix to ensure that number of array elements dont exceed the array size

fixes: e1161fa ("drivers:adc:ad713x:add ad7134 data format")

Pull Request Description

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PR Type

• [x] Bug fix (change that fixes an issue)
• [ ] New feature (change that adds new functionality)
• [ ] Breaking change (has dependencies in other repos or will cause CI to fail)

PR Checklist

• [x] I have followed the Coding style guidelines
• [x] I have performed a self-review of the changes
• [x] I have commented my code, at least hard-to-understand parts
• [x] I have build all projects affected by the changes in this PR
• [x] I have tested in hardware affected projects, at the relevant boards
• [x] I have signed off all commits from this PR
• [x] I have updated the documentation (wiki pages, ReadMe etc), if applies