iio: dac: ad5686: acquire lock when doing powerdown control

iio: dac: ad5686: acquire lock when doing powerdown control

Protect access of pwr_down_mode and pwr_down_mask fields with existing
mutex lock. Each channel exposes their own attributes for controlling
powerdown modes and powerdown state. This fixes potential race conditions
as those the write functions perform non-atomic read-modify-write
operations to those pwr_down_* fields. This issue exists since the ad5686
driver was first introduced.

Fixes: c2f37c8dcadc ("iio: dac: New driver for AD5686R, AD5685R, AD5684R Digital to analog converters")
Signed-off-by: Rodrigo Alencar <rodrigo.alencar@analog.com>
Cc: <Stable@vger.kernel.org>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>