LoraPhy: Use enum for coding rate

[maribu]

Proposed Changes

• struct member uint8_t codingRate in struct LoraTxParameters is changed to an enum

This makes reading the code a lot easier compared to the use of "magic numbers".

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 84.61%. Comparing base (1809ba2) to head (6903d55).

Additional details and impacted files

@@           Coverage Diff            @@
##           develop     #203   +/-   ##
========================================
  Coverage    84.61%   84.61%           
========================================
  Files           67       67           
  Lines         7015     7015           
========================================
  Hits          5936     5936           
  Misses        1079     1079           

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