iommu/vt-d: Use DEVICE_ATTR_RO macro

Use DEVICE_ATTR_RO() helper instead of plain DEVICE_ATTR(),
which makes the code a bit shorter and easier to read.

Signed-off-by: YueHaibing <yuehaibing@huawei.com>
Signed-off-by: Lu Baolu <baolu.lu@linux.intel.com>
Link: https://lore.kernel.org/r/20210528130229.22108-1-yuehaibing@huawei.com
Link: https://lore.kernel.org/r/20210610020115.1637656-19-baolu.lu@linux.intel.com


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