staging:iio:dac:ad5446: Do not exit powerdown when writing a sample

Both the powerdown mode bits and the sample value are stored in the same
register, so writing a sample while the device is powered down will clear the
power down bits. To avoid this only update the cached value when the device is
powered down.

Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
Acked-by: Jonathan Cameron <jic23@kernel.org>

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v1 actually had a small bug in that it would still write to the device's
register when the sample was updated while the device was powered down. This was
not critical since it would send out the powerdown mode again.
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>