regulator: tps6586x-regulator - fix value range checking for val

val is used as array index of ri->voltages.
Thus the valid value range should be 0 .. ri->desc.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>