Fix offset translation for STS servos.

[kacper-uminski]

Hi!

This fixes offset translation for Feetech STS servos (fixes #96), making it continuous from -180 to 180 degrees. One can now set a new zero position by simply setting the offset to 0, reading the current position, and writing it to the offset, which previously broke on negative positions and required one to negate positive positions.

I have tested this to work on the STS3215.

A breaking change this introduces is changing the register type for offset from i16 to u16, which changes the signature for read_raw_offset, write_raw_offset and sync_write_raw_offset to take u16 instead of i16. I feel that this is a worthwhile change, as the register is never interpreted as a signed integer, just an unsigned magnitude (0--2047) and a direction bit (bit 11). I am interpreting the direction bit as positive low and negative high.

[pierre-rouanet]

Thanks again for your contributions @kacper-uminski 💯 !