Properly handle older Z-Stack SYS.SetTxPower responses

[puddly]

Older Z-Stack releases reply to SYS.SetTxPower.Req with the set power level instead of a SUCCESS / INVALID_PARAMETER status code.

Fixes #81.

[codecov-commenter]

Codecov Report

Merging #82 (dc926ce) into dev (9bdf3ae) will increase coverage by 0.02%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##              dev      #82      +/-   ##
==========================================
+ Coverage   98.65%   98.68%   +0.02%     
==========================================
  Files          44       44              
  Lines        3717     3722       +5     
==========================================
+ Hits         3667     3673       +6     
+ Misses         50       49       -1     

Impacted Files	Coverage Δ
zigpy_znp/commands/sys.py	100.00% <ø> (ø)
zigpy_znp/config.py	100.00% <ø> (ø)
zigpy_znp/zigbee/application.py	95.70% <100.00%> (+0.21%)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 9bdf3ae...dc926ce. Read the comment docs.

[puddly]

@Tao173, are you still having this issue tube0013/tube_gateways#39? If so, can you enable ZHA debug logging (https://www.home-assistant.io/integrations/zha/#debug-logging) and post the section of the startup log right before this error?