staging: iio_simple_dummy: fix return types

The functions iio_dummy_remove(), iio_simple_dummy_events_unregister() and
iio_dummy_evgen_release_irq() were changed to return void instead of int.

Signed-off-by: Vladimirs Ambrosovs <rodriguez.twister@gmail.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>