regulator: wm8400: Adjust the equation for selector >= 15 in wm8400_ldo_list_voltage

Adjust the equation for selector >= 15 for better readability.
The equation "1700000 + ((selector - 15) * 100000)" can be interpreted by:
Starting from selector 15: Steps of 100mV from 1700mV

Signed-off-by: Axel Lin <axel.lin@gmail.com>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>