iommu/amd: Add default branch in amd_iommu_capable()

Recent gcc warns about switching on an enumeration, but not having
an explicit case statement for all members of the enumeration. To
show the compiler this is intentional, we simply add a default case
with nothing more than a break statement.

Signed-off-by: Lu Baolu <baolu.lu@linux.intel.com>
Signed-off-by: Joerg Roedel <jroedel@suse.de>