Do not get battery status for mains powered devices

[starkillerOG]

This schould fix the errors as reported here: #1121 (comment)
If the battery status is requested from a mains powered device like the lumi.plug.mmeu01 it gives an error like:
miio.exceptions.DeviceError: {'code': -4003, 'message': 'prop or action not exist'}

[starkillerOG]

@rytilahti could you look at this and merge?

[starkillerOG]

@syssi any more suggestions?

[codecov-commenter]

Codecov Report

Merging #1131 (d16dd98) into master (981368c) will decrease coverage by 0.05%.
The diff coverage is 22.22%.

@@            Coverage Diff             @@
##           master    #1131      +/-   ##
==========================================
- Coverage   74.87%   74.81%   -0.06%     
==========================================
  Files          76       76              
  Lines        8999     9006       +7     
  Branches      767      769       +2     
==========================================
  Hits         6738     6738              
- Misses       2077     2084       +7     
  Partials      184      184              

Impacted Files	Coverage Δ
miio/gateway/devices/subdevice.py	36.87% <22.22%> (-1.93%)	⬇️

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[syssi]

I will merge this PR later the day. I want to give @rytilahti the chance to react.

[starkillerOG]

I think this can now be merged