regulator: palmas: Remove wrong comment for the equation calculating num_voltages

Current equation on the comment is wrong.
For linear mapping starting from 0, the equation is (maxV-minV)/stepV + 1.
Since the linear mapping for PALMAS is not all starting from 0, the equation
on the comment is not useful and misleading. Thus remove it.

Signed-off-by: Axel Lin <axel.lin@ingics.com>
Signed-off-by: Mark Brown <broonie@linaro.org>