ads1x15: style improvement (#315)
ads1x15: style improvement (#315)

No functional change.

- Move private globals at the bottom of the file.
- Move ads1x15AnalogPin close to its methods.
- Stop using named return variable.
- Inline newADS1x15.
- Move gainConfig and gainVoltage as global variables.
- Wraps comments.
- Rename mutex to mu.
- Use -math.MaxInt16 instead of math.MinInt16 because of the voltage multiplier.
- Change error strings to start with a lower case.
google/periph