intel-iommu: Cope with broken HP DC7900 BIOS

Yet another reason why trusting this stuff to the BIOS was a bad idea.
The HP DC7900 BIOS reports an iommu at an address which just returns all
ones, when VT-d is disabled in the BIOS.

Fix up the missing iounmap in the error paths while we're at it.

Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>