x86, AMD IOMMU: add an emergency exit to the completion wait loop

To make the loop waiting for the completion wait command not wait forever this
patch adds a limit of cycles that loop.

Signed-off-by: Joerg Roedel <joerg.roedel@amd.com>
Cc: iommu@lists.linux-foundation.org
Cc: bhavna.sarathy@amd.com
Cc: robert.richter@amd.com
Signed-off-by: Ingo Molnar <mingo@elte.hu>