staging:iio:dac/dds/impedance dev_info to indio_dev for consistency

We had a random missmatch of these two.  Lets pick the most common
and get rid of the other.

Signed-off-by: Jonathan Cameron <jic23@cam.ac.uk>

For adxxxx parts
Acked-by: Michael Hennerich <michael.hennerich@analog.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>