iommu/arm-smmu-v3: Fix fall-through warning for Clang

Fix the following fallthrough warning (arm64-randconfig with Clang):

drivers/iommu/arm/arm-smmu-v3/arm-smmu-v3.c:382:2: warning: unannotated fall-through between switch labels [-Wimplicit-fallthrough]
Reported-by: kernel test robot <lkp@intel.com>
Link: https://lore.kernel.org/lkml/60edca25.k00ut905IFBjPyt5%25lkp@intel.com/Signed-off-by: Gustavo A. R. Silva <gustavoars@kernel.org>

drivers/iommu/arm/arm-smmu-v3/arm-smmu-v3.c

@@ -379,6 +379,7 @@ static void arm_smmu_cmdq_skip_err(struct arm_smmu_device *smmu)
 	switch (idx) {
 	case CMDQ_ERR_CERROR_ABT_IDX:
 		dev_err(smmu->dev, "retrying command fetch\n");
+		return;
 	case CMDQ_ERR_CERROR_NONE_IDX:
 		return;
 	case CMDQ_ERR_CERROR_ATC_INV_IDX: