staging:iio:ring_sw don't provide read last function.

No longer needed as we don't have drivers providing sysfs access
to buffered data.

Signed-off-by: Jonathan Cameron <jic23@kernel.org>
Acked-by: Lars-Peter Clausen <lars@metafoo.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>