iommu/amd: Fix compiler warning in copy_device_table()

This was reported by the kbuild bot. The condition in which
entry would be used uninitialized can not happen, because
when there is no iommu this function would never be called.
But its no fast-path, so fix the warning anyway.

Reported-by: kbuild test robot <fengguang.wu@intel.com>
Acked-by: Baoquan He <bhe@redhat.com>
Signed-off-by: Joerg Roedel <jroedel@suse.de>