iommu/arm-smmu: Mark expected switch fall-through

Now that -Wimplicit-fallthrough is passed to GCC by default, the
following warning shows up:

../drivers/iommu/arm-smmu-v3.c: In function ‘arm_smmu_write_strtab_ent’:
../drivers/iommu/arm-smmu-v3.c:1189:7: warning: this statement may fall
 through [-Wimplicit-fallthrough=]
    if (disable_bypass)
       ^
../drivers/iommu/arm-smmu-v3.c:1191:3: note: here
   default:
   ^~~~~~~

Rework so that the compiler doesn't warn about fall-through. Make it
clearer by calling 'BUG_ON()' when disable_bypass is set, and always
'break;'
Signed-off-by: Anders Roxell <anders.roxell@linaro.org>
Acked-by: Will Deacon <will@kernel.org>
Signed-off-by: Joerg Roedel <jroedel@suse.de>

drivers/iommu/arm-smmu-v3.c

@@ -1186,7 +1186,7 @@ static void arm_smmu_write_strtab_ent(struct arm_smmu_master *master, u32 sid,
 			ste_live = true;
 			break;
 		case STRTAB_STE_0_CFG_ABORT:
-			if (disable_bypass)
+			BUG_ON(!disable_bypass);
 			break;
 		default:
 			BUG(); /* STE corruption */