iio: dac: Convert powerdown read callbacks to sysfs_emit()

iio: dac: Convert powerdown read callbacks to sysfs_emit()

Update DAC drivers powerdown attribute show callback to use the new
sysfs_emit() function.

sysfs_emit() is preferred over raw s*printf() for sysfs attributes since it
knows about the sysfs buffer specifics and has some built-in sanity checks.

Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
Link: https://lore.kernel.org/r/20210320071405.9347-5-lars@metafoo.de
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>