iio: dac: ad5446: Make use of the cleanup helpers

iio: dac: ad5446: Make use of the cleanup helpers

Use the auto unlocking helpers from cleanup.h. Allows for some code
simplification.

While at it, don't use the ternary operator in
ad5446_write_dac_powerdown() and add an helper function to write the DAC
code. The reason for the function was purely to avoid having to use
unreachable().

Signed-off-by: Nuno Sá <nuno.sa@analog.com>
Reviewed-by: Andy Shevchenko <andriy.shevchenko@intel.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>