Incorrect calculation for ReadLoad

[BrettRD]

Hello there! Thanks for the library

I tried to make a simple friction model, but the ReadLoad method here isn't producing signed values like other implementations do

This lib isn't using the high byte STS_PRESENT_LOAD_H, I think it's just a matter of using a two-byte read instead, but I've seen some other code manually handling the sign bit.

I'll have a hack on it later and make a PR

Cheers