mlx4_core: Don't leak mailbox for SET_PORT on Ethernet ports

Commit 793730bf ("mlx4_core: Don't perform SET_PORT command for
Ethernet ports") introduced a leak of mailbox buffers when SET_PORT
was called for Ethernet ports, since it added a return after the
mailbox was allocated.  Fix this by checking the port type and
returning *before* allocating the mailbox.
Signed-off-by: Roland Dreier <rolandd@cisco.com>

drivers/net/mlx4/port.c

@@ -299,13 +299,14 @@ int mlx4_SET_PORT(struct mlx4_dev *dev, u8 port)
 	struct mlx4_cmd_mailbox *mailbox;
 	int err;
 
+	if (dev->caps.port_type[port] == MLX4_PORT_TYPE_ETH)
+		return 0;
+
 	mailbox = mlx4_alloc_cmd_mailbox(dev);
 	if (IS_ERR(mailbox))
 		return PTR_ERR(mailbox);
 
 	memset(mailbox->buf, 0, 256);
-	if (dev->caps.port_type[port] == MLX4_PORT_TYPE_ETH)
-		return 0;
 
 	((__be32 *) mailbox->buf)[1] = dev->caps.ib_port_def_cap[port];
 	err = mlx4_cmd(dev, mailbox->dma, port, 0, MLX4_CMD_SET_PORT,