regulator: wm8400: Use regulator_map_voltage_linear for wm8400_dcdc_ops

wm8400_dcdc_ops uses simple linear voltage maps.
Thus use regulator_map_voltage_linear is more efficient than using the default
regulator_map_voltage_iterate.
Signed-off-by: Axel Lin <axel.lin@gmail.com>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>

drivers/regulator/wm8400-regulator.c

@@ -152,6 +152,7 @@ static struct regulator_ops wm8400_dcdc_ops = {
 	.enable = regulator_enable_regmap,
 	.disable = regulator_disable_regmap,
 	.list_voltage = regulator_list_voltage_linear,
+	.map_voltage = regulator_map_voltage_linear,
 	.get_voltage_sel = regulator_get_voltage_sel_regmap,
 	.set_voltage_sel = regulator_set_voltage_sel_regmap,
 	.get_mode = wm8400_dcdc_get_mode,