Use ttnpb.DataRateIndex enum where possible

[rvolosatovs]

Summary

Use ttnpb.DataRateIndex enum where possible

Why do we need this?

For clarity and correctness

What is already there? What do you see now?

Since #2380 it seems that all band.Band fields use ttnpb.DataRateIndex instead of untyped numerical types to represent data rate indexes

What is missing? What do you want to see?

• Refactor frequencyplans to use ttnpb.DataRateIndex
• Refactor any other package if there's still untyped numerical types used to represent data rate indexes

How do you propose to implement this?

Change types to ttnpb.DataRateIndex where applicable

Can you do this yourself and submit a Pull Request?

yes