iommu/virtio: Clean up bus_set_iommu()

Stop calling bus_set_iommu() since it's now unnecessary, and simplify
the probe failure path accordingly.

Reviewed-by: Jean-Philippe Brucker <jean-philippe@linaro.org>
Signed-off-by: Robin Murphy <robin.murphy@arm.com>
Link: https://lore.kernel.org/r/0ff6f9166081724510e6772e43d45b317cab8c58.1660572783.git.robin.murphy@arm.com


Signed-off-by: Joerg Roedel <jroedel@suse.de>