iommu/rockchip: Fix error handling in attach

Currently if the driver encounters an error while attaching device, it
will leave the IOMMU in an inconsistent state. Even though it shouldn't
really happen in reality, let's just add proper error path to keep
things consistent.

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