xen/arm: smmuv3: Use fallthrough pseudo-keyword

Backport commit df561f6688fef775baa341a0f5d960becd248b11
"treewide: Use fallthrough pseudo-keyword" from Linux kernel.

Replace the existing /* fall through */ comments and its variants with
the new pseudo-keyword macro fallthrough[1]. Also, remove unnecessary
fall-through markings when it is the case.

Signed-off-by: Rahul Singh <rahul.singh@arm.com>
Reviewed-by: Bertrand Marquis <bertrand.marquis@arm.com>
Reviewed-by: Stefano Stabellini <sstabellini@kernel.org>

xen/drivers/passthrough/arm/smmu-v3.c

@@ -932,7 +932,6 @@ static void arm_smmu_cmdq_skip_err(struct arm_smmu_device *smmu)
 		 */
 		return;
 	case CMDQ_ERR_CERROR_ILL_IDX:
-		/* Fallthrough */
 	default:
 		break;
 	}
@@ -2488,7 +2487,7 @@ static int arm_smmu_device_hw_probe(struct arm_smmu_device *smmu)
 	switch (FIELD_GET(IDR0_STALL_MODEL, reg)) {
 	case IDR0_STALL_MODEL_FORCE:
 		smmu->features |= ARM_SMMU_FEAT_STALL_FORCE;
-		/* Fallthrough */
+		fallthrough;
 	case IDR0_STALL_MODEL_STALL:
 		smmu->features |= ARM_SMMU_FEAT_STALLS;
 	}
@@ -2505,7 +2504,7 @@ static int arm_smmu_device_hw_probe(struct arm_smmu_device *smmu)
 	switch (FIELD_GET(IDR0_TTF, reg)) {
 	case IDR0_TTF_AARCH32_64:
 		smmu->ias = 40;
-		/* Fallthrough */
+		fallthrough;
 	case IDR0_TTF_AARCH64:
 		break;
 	default:
@@ -2589,7 +2588,7 @@ static int arm_smmu_device_hw_probe(struct arm_smmu_device *smmu)
 	default:
 		dev_info(smmu->dev,
 			"unknown output address size. Truncating to 48-bit\n");
-		/* Fallthrough */
+		fallthrough;
 	case IDR5_OAS_48_BIT:
 		smmu->oas = 48;
 	}