xbee: fix updating values for PWM and OnOff clusters

[Shulyaka]

remote_at returns None on successful value set, and an exception on error. The current code assumes return of SUCCESS on success and other values on error, which is not correct. Fixing that.

[codecov-commenter]

Codecov Report

Merging #1096 (6c2ae78) into dev (e4b8eda) will increase coverage by 0.07%.
The diff coverage is 0.00%.

@@            Coverage Diff             @@
##              dev    #1096      +/-   ##
==========================================
+ Coverage   80.68%   80.76%   +0.07%     
==========================================
  Files         197      197              
  Lines        5308     5303       -5     
==========================================
  Hits         4283     4283              
+ Misses       1025     1020       -5     

Impacted Files	Coverage Δ
zhaquirks/xbee/__init__.py	44.05% <0.00%> (+1.06%)	⬆️

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[coveralls]

Pull Request Test Coverage Report for Build 1376615272

• 0 of 6 (0.0%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.08%) to 80.766%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
zhaquirks/xbee/init.py	0	6	0.0%

Totals
Change from base Build 1375751760:	0.08%
Covered Lines:	4283
Relevant Lines:	5303

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💛 - Coveralls