niu: Fix error checking in niu_ethflow_to_class.

The callers of niu_ethflow_to_class expect zero as error, but it returns
-1 instead.

Signed-off-by: Andreas Schwab <schwab@suse.de>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/niu.c

@@ -6417,7 +6417,7 @@ static int niu_ethflow_to_class(int flow_type, u64 *class)
 		*class = CLASS_CODE_SCTP_IPV6;
 		break;
 	default:
-		return -1;
+		return 0;
 	}
 
 	return 1;