BatteryMonitor: add casts to make new compiler versions happy

This will just remove the warnings below
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/home/khautio/src/AeroQuad/AeroQuad32/../AeroQuad/AeroQuad.ino:1019:55: warning: narrowing conversion of '2.53e+3f' from 'float' to 'short int' inside { } is ill-formed in C++11 [-Wnarrowing]
/home/khautio/src/AeroQuad/AeroQuad32/../AeroQuad/AeroQuad.ino:1019:55: warning: narrowing conversion of '0.0f' from 'float' to 'short int' inside { } is ill-formed in C++11 [-Wnarrowing]
/home/khautio/src/AeroQuad/AeroQuad32/../AeroQuad/AeroQuad.ino:1019:55: warning: narrowing conversion of '0.0f' from 'float' to 'short int' inside { } is ill-formed in C++11 [-Wnarrowing]
/home/khautio/src/AeroQuad/AeroQuad32/../AeroQuad/AeroQuad.ino:1019:55: warning: narrowing conversion of '0.0f' from 'float' to 'short int' inside { } is ill-formed in C++11 [-Wnarrowing]
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