iommu/arm-smmu: Axe a useless test in 'arm_smmu_master_alloc_smes()'

'iommu_group_get_for_dev()' never returns NULL, so this test can be removed.

Reviewed-by: Robin Murphy <robin.murphy@arm.com>
Signed-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Signed-off-by: Will Deacon <will@kernel.org>