net/mlx4_core: fix handling return value of mlx4_slave_convert_port

The function can return negative values, so its result should
be assigned to signed variable.

The problem has been detected using proposed semantic patch
scripts/coccinelle/tests/assign_signed_to_unsigned.cocci [1].

[1]: http://permalink.gmane.org/gmane.linux.kernel/2046107

Fixes: fc48866f7 ('net/mlx4: Adapt code for N-Port VF')
Signed-off-by: Andrzej Hajda <a.hajda@samsung.com>
Acked-by: Or Gerlitz <ogerlitz@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/ethernet/mellanox/mlx4/resource_tracker.c

@@ -4306,9 +4306,10 @@ int mlx4_QP_FLOW_STEERING_ATTACH_wrapper(struct mlx4_dev *dev, int slave,
 		return -EOPNOTSUPP;
 
 	ctrl = (struct mlx4_net_trans_rule_hw_ctrl *)inbox->buf;
-	ctrl->port = mlx4_slave_convert_port(dev, slave, ctrl->port);
-	if (ctrl->port <= 0)
+	err = mlx4_slave_convert_port(dev, slave, ctrl->port);
+	if (err <= 0)
 		return -EINVAL;
+	ctrl->port = err;
 	qpn = be32_to_cpu(ctrl->qpn) & 0xffffff;
 	err = get_res(dev, slave, qpn, RES_QP, &rqp);
 	if (err) {