iommu/vt-d: Fix crash when accessing VT-d sysfs entries

The link between the iommu sysfs-device and the struct
intel_iommu is no longer stored as driver-data. Update the
code to use the new access method.

Reported-by: Dave Jones <davej@codemonkey.org.uk>
Fixes: 39ab9555

 ('iommu: Add sysfs bindings for struct iommu_device')
Signed-off-by: Joerg Roedel <jroedel@suse.de>