iommu/dart: Clean up bus_set_iommu()

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

Tested-by: Sven Peter <sven@svenpeter.dev>
Reviewed-by: Sven Peter <sven@svenpeter.dev>
Signed-off-by: Robin Murphy <robin.murphy@arm.com>
Link: https://lore.kernel.org/r/afe138964196907d58147a686c1dcd6a12f9e210.1660572783.git.robin.murphy@arm.com


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