staging:iio: rip out scan_el attributes. Now handled as iio_dev_attrs like everything else.

Drivers have no need to use this functionality any more and we save a lot of
code by getting rid of it.

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