staging:iio:core:naming: 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.  This patch covers the core.  Others
will clean up the drivers.

Signed-off-by: Jonathan Cameron <jic23@cam.ac.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>