'iommu_group_get_for_dev()' never returns NULL, so this test can be
simplified a bit.

This way, the test is consistent with all other calls to
'iommu_group_get_for_dev()'.
Signed-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Reviewed-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Joerg Roedel <jroedel@suse.de>