TWBR register should not be hardcoded

The TWBR register value should not be hardcoded, as it will not work as intended on devices running at another frequency than 16MHz.

For instance the I2C frequency of a microcontroller running at 8MHz would only be 200kHz:
```
(8 * 10^6) / ((12 * 2) + 16) = 200000
```

The line is actually just taken from the Arduino source code: https://github.com/arduino/Arduino/blob/master/libraries/Wire/utility/twi.c#L82.