regulator: Fix off-by-one value range checking for mc13xxx_regulator_get_voltage

We use val as array index,
thus the valid value rangae for val should be 0 .. n_voltages-1.

Signed-off-by: Axel Lin <axel.lin@gmail.com>
Acked-by: Mark Brown <broonie@opensource.wolfsonmicro.com>
Signed-off-by: Liam Girdwood <lrg@slimlogic.co.uk>