regulator: max77686: Use regulator_map_voltage_linear for simple linear mappings

Both max77686_ops and max77686_buck_dvs_ops use simple linear voltage maps.
Thus use regulator_map_voltage_linear is more efficient than using the defult
regulator_map_voltage_iterate.

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