staging:iio:dac:ad5791: Convert attributes to new naming spec

Add the missing "voltage" chan_type to the powerdown attributes.

Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
Acked-by: Jonathan Cameron <jic23@cam.ac.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>