regulator: ab8500 - fix off-by-one value range checking for selector

selector is used as array index of info->supported_voltages
Thus the valid value range should be 0 .. info->voltages_len -1

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