staging:iio:adc:max1363 fix missing update_scan_mask callback.

When moving over to the new sw_ring_preenable I managed to add
this callback to only one of the two iio_info structures. As
such only some devices will currently work.

Signed-off-by: Jonathan Cameron <jic23@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>