iommu/rockchip: Fix error handling in init

It's hard to undo bus_set_iommu() in the error path, so move it to the
end of rk_iommu_probe().

Signed-off-by: Jeffy Chen <jeffy.chen@rock-chips.com>
Reviewed-by: Tomasz Figa <tfiga@chromium.org>
Reviewed-by: Robin Murphy <robin.murphy@arm.com>
Signed-off-by: Joerg Roedel <jroedel@suse.de>