regulator: Fix the logic to ensure new voltage setting in valid range

I think this is a typo.
To ensure new voltage setting won't greater than desc->max,
the equation should be desc->min + desc->step * new_val <= desc->max.

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