staging:iio:meter:ade7758: Use private data space from iio_allocate_device

Use private data space from iio_allocate_device.
Drop dev_data in favor of iio_priv().
Fix indention issues from previous patches.

Signed-off-by: Michael Hennerich <michael.hennerich@analog.com>
Acked-by: Jonathan Cameron <jic23@cam.ac.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>