intel-iommu: Make "Unknown DMAR structure" message more informative

We might as well print the type of the DMAR structure we don't know how
to handle when skipping it.  Then someone getting this message has a
chance of telling whether the structure is just bogus, or if there
really is something valid that the kernel doesn't know how to handle.

Signed-off-by: Roland Dreier <rolandd@cisco.com>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>