iio: dummy_evgen: Fix use after free on error in iio_dummy_evgen_create()

We need to preserve the "iio_evgen->irq_sim_domain" error code before
we free "iio_evgen" otherwise it leads to a use after free.

Fixes: 337cbeb2 ("genirq/irq_sim: Simplify the API")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Marc Zyngier <maz@kernel.org>