ab3100: fix off-by-one value range checking for voltage selector

We use voltage selector as an array index for typ_voltages.
Thus the valid range for voltage selector should be 0..voltages_len-1.

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