regulator: keep index within bounds in da9034_get_ldo12_voltage()

If selector equals ARRAY_SIZE(da9034_ldo12_data), that is one too
large already.

Signed-off-by: Roel Kluin <roel.kluin@gmail.com>
Acked-by: Mark Brown <broonie@opensource.wolfsonmicro.com>
Signed-off-by: Liam Girdwood <lrg@slimlogic.co.uk>