Fix bug and buffer overrun in modbusExchangeRegisters (#5)
Fix bug and buffer overrun in modbusExchangeRegisters (#5)

The third argument function intToModbusRegisters is a number of modbus
registers, while the call in modbusExchangeRegisters provided the
number of bytes instead. With an rxbuffer of 256 bytes, reading 64
registers or more would lead to a buffer overrun.
mbs38/yaMBSiavr