regulator: ti-abb: Fix off-by-one valid range checking for abb->current_info_idx

abb->current_info_idx is used as array subscript to access volt_table,
thus the valid value range should be 0 ... desc->n_voltages - 1.

Signed-off-by: Axel Lin <axel.lin@ingics.com>
Acked-by: Nishanth Menon <nm@ti.com>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>