net/mlx5e: Fix an error code in mlx5e_arfs_create_tables()

When the code execute 'if (!priv->fs.arfs->wq)', the value of err is 0.
So, we use -ENOMEM to indicate that the function
create_singlethread_workqueue() return NULL.

Clean up smatch warning:
drivers/net/ethernet/mellanox/mlx5/core/en_arfs.c:373
mlx5e_arfs_create_tables() warn: missing error code 'err'.

Reported-by: Abaci Robot <abaci@linux.alibaba.com>
Fixes: f6755b80 ("net/mlx5e: Dynamic alloc arfs table for netdev when needed")
Signed-off-by: Yang Li <yang.lee@linux.alibaba.com>
Signed-off-by: Saeed Mahameed <saeedm@nvidia.com>