iommu: Make iommu_bus_notifier return NOTIFY_DONE rather than error code

In iommu_bus_notifier(), when action is
BUS_NOTIFY_ADD_DEVICE, it will return 'ops->add_device(dev)'
directly. But ops->add_device will return ERR_VAL, such as
-ENODEV. These value will make notifier_call_chain() not to
traverse the remain nodes in struct notifier_block list.

This patch revises iommu_bus_notifier() to return
NOTIFY_DONE when some errors happened in ops->add_device().

Signed-off-by: zhichang.yuan <yuanzhichang@hisilicon.com>
Signed-off-by: Joerg Roedel <jroedel@suse.de>