airpurifier_miot: return OperationMode.Unknown if mode is unknown

[rytilahti]

Fixes #1106

[codecov-commenter]

Codecov Report

Merging #1111 (ed0093f) into master (924cb07) will decrease coverage by 0.01%.
The diff coverage is 57.14%.

@@            Coverage Diff             @@
##           master    #1111      +/-   ##
==========================================
- Coverage   76.07%   76.05%   -0.02%     
==========================================
  Files          75       75              
  Lines        8722     8728       +6     
  Branches      747      747              
==========================================
+ Hits         6635     6638       +3     
- Misses       1905     1908       +3     
  Partials      182      182              

Impacted Files	Coverage Δ
miio/airpurifier_miot.py	87.93% <57.14%> (-1.36%)	⬇️
miio/discovery.py	48.61% <0.00%> (ø)

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