Fix native_msg calibration bug

[sjoro]

Fix a bug in get_dataset-method in native_msg.py. Calibrated dataset values were accidentally set back to counts.

• [ x] Passes git diff origin/develop **/*py | flake8 --diff

[coveralls]

Coverage increased (+0.03%) to 65.124% when pulling b455e2c on sjoro:develop into 7490ccf on pytroll:develop.

[codecov-io]

Codecov Report

Merging #285 into develop will increase coverage by 0.03%.
The diff coverage is n/a.

@@             Coverage Diff             @@
##           develop     #285      +/-   ##
===========================================
+ Coverage    65.09%   65.12%   +0.03%     
===========================================
  Files          115      115              
  Lines        14088    14087       -1     
===========================================
+ Hits          9170     9174       +4     
+ Misses        4918     4913       -5

Impacted Files	Coverage Δ
satpy/readers/native_msg.py	18.55% <ø> (+0.09%)	⬆️
satpy/scene.py	82.75% <ø> (ø)	⬆️
satpy/readers/__init__.py	94.53% <0%> (+1.68%)	⬆️

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[mraspaud]

You don't need the changes in scene.py, please remove them.

[sjoro]

Changes in scene.py removed.

[mraspaud]

lgtm