net/mlx5: Fix a NULL vs IS_ERR() check

The mlx5_esw_offloads_devlink_port() function returns error pointers, not
NULL.

Fixes: 7bef147a ("net/mlx5: Don't skip vport check")
Signed-off-by: Dan Carpenter <dan.carpenter@linaro.org>
Reviewed-by: Wojciech Drewek <wojciech.drewek@intel.com>
Signed-off-by: Saeed Mahameed <saeedm@nvidia.com>