max8998: fix off-by-one value range checking

In max8998_list_voltage() and max8998_set_voltage(),
we use ldo as array index of ldo_voltage_map.
Thus the valid range should be 0 .. ARRAY_SIZE(ldo_voltage_map)-1.

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