iommu/amd: Fix crash when accessing AMD-Vi sysfs entries

The link between the iommu sysfs-device and the struct
amd_iommu is no longer stored as driver-data. Update the
code to the new correct way of getting from device to
amd_iommu.

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>