Fix writing network settings with missing TX power

[puddly]

EZSP is the only radio type that expects an explicit TX power to be supplied during network formation. We really should migrate all of these radio library methods to underscore-prefixed variants (e.g. write_network_info to _write_network_info) so that zigpy can fix settings up before having the radio library perform the operation.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.55%. Comparing base (02d7870) to head (a917be8).
⚠️ Report is 1 commits behind head on dev.

Additional details and impacted files

@@           Coverage Diff           @@
##              dev     #695   +/-   ##
=======================================
  Coverage   99.55%   99.55%           
=======================================
  Files          58       58           
  Lines        4039     4040    +1     
=======================================
+ Hits         4021     4022    +1     
  Misses         18       18           

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[TheJulianJES]

Looks good