iio: dac: ad5686: add helpers to handle powerdown masks

iio: dac: ad5686: add helpers to handle powerdown masks

Add ad5686_pd_field_set() and ad5686_pd_field_get() helpers to cleanup
powerdown mask control. Define AD5686_PD_* constants, e.g. AD5686_PD_MSK
to hold powerdown mask value for a single channel. AD5686_LDAC_PWRDN_*
macros are replaced by AD5686_PD_MODE_*, because they are unused and the
LDAC feature for async load of DAC channel values is not related to power
down control.

Signed-off-by: Rodrigo Alencar <rodrigo.alencar@analog.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>