VT-d: conditionalize IOTLB register offset check

VT-d: conditionalize IOTLB register offset check

As of commit 6773b1a7584a ("VT-d: Don't assume register-based
invalidation is always supported") we don't (try to) use register based
invalidation anymore when that's not supported by hardware. Hence
there's also no point in the respective check, avoiding pointless IOMMU
initialization failure. After all the spec (version 3.3 at the time of
writing) doesn't say what the respective Extended Capability Register
field would contain in such a case.

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Kevin Tian <kevin.tian@intel.com>