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

commit f55205f4d4a8823a11bb8b37ef2ecbd78fb09463 upstream.

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>
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>