iio: dac: ad5686: fix powerdown control on dual-channel devices

iio: dac: ad5686: fix powerdown control on dual-channel devices

Fix powerdown control by using a proper bit shift for the powerdown mask
values. During initialization, powerdown bits are initialized so that
unused bits are set to 1 and the correct bit shift is used. Dual-channel
devices use one-hot encoding in the address and that reflects on the
position of the powerdown bits, which are not channel-index based
for that case. Quad-channel devices also use one-hot encoding for the
channel address but the result of log2(address) coincides with the channel
index value. Mask as 0x3U is used rather than 0x3, because shift can reach
value of 30 (last channel of a 16-channel device), which would mess with
the sign bit. The issue was introduced when first adding support for
dual-channel devices, which overlooked powerdown control differences.

Fixes: 7dc8faeab3e3 ("iio: dac: ad5686: add support for AD5338R")
Signed-off-by: Rodrigo Alencar <rodrigo.alencar@analog.com>
Cc: <Stable@vger.kernel.org>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>