x86/amd: iommu_set_device_table() must not be __init

This function is called from enable_iommus(), which in turn is used
from amd_iommu_resume().

Cc: stable@vger.kernel.org
Signed-off-by: Jan Beulich <jbeulich@suse.com>
Signed-off-by: Joerg Roedel <joerg.roedel@amd.com>