Modbus callback register offset incorrect

[HowardBray1966]

If a FC16 modbus pdu is received with register set to 0 (as proven with Wireshark), the setValues callback is called with register +1.

[janiversen]

please read the documentation! this is a difference between the addresses (network) and representation (python internal)

feel free to submit a PR if you want the dev branch to work differently.