staging: iio: sca3000 fix bug due to scan_element directory move.

The device found passed to the attr functions is that of the ring
buffer, not the the iio_dev so we need to bounce through one more
hop to get the right address.

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