iommu/amd: Missing error code in amd_iommu_init_device()

We should set "ret" to -EINVAL if iommu_group_get() fails.

Fixes: 55c99a4d ("iommu/amd: Use iommu_attach_group()")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Joerg Roedel <jroedel@suse.de>