staging:iio:adc:ad7298: Convert to new channel registration method

Convert to new channel registration method Update
Add missing call to iio_trigger_notify_done()

Backported to a point that will allow the driver to build all the
way through the series.

V2: IIO_CHAN macro usage updated.

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