regulator: core: Fix logic to determinate if regulator can change voltage

Having a linear_min_sel setting means the first linear_min_sel selectors are
invalid. We need to subtract linear_min_sel when use n_voltages to determinate
if regulator can change voltage.

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