Ensure vendor table key is upper case

[kengu]

See #178.

[codecov]

Codecov Report

Attention: 5 lines in your changes are missing coverage. Please review.

Comparison is base (5de4ba0) 76.82% compared to head (4332cd4) 76.39%.

Files	Patch %	Lines
lib/src/device_info/vendor_table.dart	0.00%	5 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##              dev     #179      +/-   ##
==========================================
- Coverage   76.82%   76.39%   -0.43%     
==========================================
  Files          19       19              
  Lines         535      538       +3     
==========================================
  Hits          411      411              
- Misses        124      127       +3     

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