net/mlx5e: Remove incorrect debugfs_create_dir NULL check in hairpin

Remove the NULL check on debugfs_create_dir() return value as the
function returns an ERR pointer on failure, not NULL.
The check is not replaced with a IS_ERR_OR_NULL() as
debugfs_create_file(), and debugfs functions in general don't need error
checking.

Fixes: 0e414518 ("net/mlx5e: Add hairpin debugfs files")
Reported-by: Dan Carpenter <error27@gmail.com>
Signed-off-by: Gal Pressman <gal@nvidia.com>
Reviewed-by: Tariq Toukan <tariqt@nvidia.com>
Signed-off-by: Saeed Mahameed <saeedm@nvidia.com>