regulator: wm8400: Simplify set REGULATOR_MODE_IDLE mode in wm8400_dcdc_set_mode

To set REGULATOR_MODE_IDLE mode, what we do is to clear DC1_ACTIVE and
DC1_SLEEP bits, this can be done in one wm8400_set_bits() call.

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