s390: Fix fall-through warnings for Clang

Fix the following fallthrough warnings:

drivers/s390/net/ctcm_fsms.c:1457:2: warning: unannotated fall-through between switch labels [-Wimplicit-fallthrough]
drivers/s390/net/qeth_l3_main.c:437:3: warning: unannotated fall-through between switch labels [-Wimplicit-fallthrough]
drivers/s390/char/tape_char.c:374:4: warning: unannotated fall-through between switch labels [-Wimplicit-fallthrough]
arch/s390/kernel/uprobes.c:129:2: warning: unannotated fall-through between switch labels [-Wimplicit-fallthrough]

Reported-by: Nathan Chancellor <[e-mail]>
Signed-off-by: Gustavo A. R. Silva <[e-mail]>

arch/s390/kernel/uprobes.c

···
 	case DIE_SSTEP:
 		if (uprobe_post_sstep_notifier(regs))
 			return NOTIFY_STOP;
+		break;
 	default:
 		break;
 	}

drivers/s390/char/tape_char.c

···
 			case MTSEEK:
 				if (device->required_tapemarks)
 					tape_std_terminate_write(device);
-			default:
-				;
 		}
 		rc = tape_mtop(device, op.mt_op, op.mt_count);
 

drivers/s390/net/ctcm_fsms.c

···
 				get_ccwdev_lock(ch->cdev), saveflags);
 		if (rc != 0)
 			ctcm_ccw_check_rc(ch, rc, "normal RX");
+		break;
 	default:
 		break;
 	}

drivers/s390/net/qeth_l3_main.c

···
 			if (qeth_is_ipafunc_supported(card, prot,
 						      IPA_OSA_MC_ROUTER))
 				return 0;
+			goto out_inval;
 		default:
 			goto out_inval;
 		}