net/mlx5: add missing return value check

net/mlx5: add missing return value check

This patch fixes the build failure with message:
  drivers/net/mlx5/mlx5_ethdev.c: In function ‘mlx5_sysfs_switch_info’:
  drivers/net/mlx5/mlx5_ethdev.c:1381:3:
    error: ignoring return value of ‘fscanf’, declared with attribute
           warn_unused_result [-Werror=unused-result]
  fscanf(file, "%s", port_name);
    ^

Which reproduces on Ubuntu 16.04 LTS with
gcc (Ubuntu 5.4.0-6ubuntu1~16.04.10) 5.4.0 20160609.

Fixes: b2f3a3810125 ("net/mlx5: support new representor naming format")

Signed-off-by: Ali Alnubani <alialnu@mellanox.com>
Acked-by: Viacheslav Ovsiienko <viacheslavo@mellanox.com>
Acked-by: Dekel Peled <dekelp@mellanox.com>