staging: iio: max1363 add _index attribute registration

Also fixes a missing NULL to terminate one of the attribute arrays.

Signed-off-by: Jonathan Cameron <jic23@cam.ac.uk>
Signed-off-by: Manuel Stahl <manuel.stahl@iis.fraunhofer.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>