net/mlx4_en: Using positive error value for unsigned

Using a positive value for error: MLX4_NET_TRANS_RULE_NUM instead
of -EPROTONOSUPPORT, to remove compilation warning.

Signed-off-by: Eyal Perry <eyalpe@mellanox.com>
Signed-off-by: Amir Vadai <amirv@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>