Use xbee endpoint as source if set

[prairiesnpr]

trafficstars

When using an xbee as end device, the source endpoint needs to match the destination endpoint, (0XE6 or 0XE8) this will retain the source endpoint instead of overriding it.

[codecov-commenter]

Codecov Report

Base: 98.60% // Head: 98.57% // Decreases project coverage by -0.02% :warning:

Coverage data is based on head (0777cc7) compared to base (824c2b2). Patch coverage: 66.66% of modified lines in pull request are covered.

Additional details and impacted files

@@            Coverage Diff             @@
##              dev     #175      +/-   ##
==========================================
- Coverage   98.60%   98.57%   -0.03%     
==========================================
  Files          43       43              
  Lines        3865     3871       +6     
==========================================
+ Hits         3811     3816       +5     
- Misses         54       55       +1     

Impacted Files	Coverage Δ
zigpy_znp/zigbee/application.py	95.08% <50.00%> (-0.41%)	:arrow_down:
zigpy_znp/const.py	100.00% <100.00%> (ø)
zigpy_znp/api.py	97.15% <0.00%> (+0.23%)	:arrow_up:

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[prairiesnpr]

Closing in favor of https://github.com/zigpy/zigpy-znp/pull/181