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>
diff --git a/drivers/net/niu.c b/drivers/net/niu.c
index 8ee7d7b..e4765b7 100644
--- a/drivers/net/niu.c
+++ b/drivers/net/niu.c
@@ -6417,7 +6417,7 @@
 		*class = CLASS_CODE_SCTP_IPV6;
 		break;
 	default:
-		return -1;
+		return 0;
 	}
 
 	return 1;