regulator: Set list_voltage callback for max8925

Current code implements max8925_list_voltage but does not set the list_voltage
callback function in max8925_regulator_[ldo|sdv]_ops. Fix it.

Signed-off-by: Axel Lin <axel.lin@gmail.com>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>