regulator: wm8350: Use regulator_map_voltage_linear for wm8350_dcdc_ops

wm8350_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>