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

The mlx5_chains_get_table() function doesn't return NULL, it returns
error pointers so we need to fix this condition.

Fixes: 34ca6535

 ("net/mlx5: E-Switch, Indirect table infrastructure")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Reviewed-by: Vlad Buslov <vladbu@nvidia.com>
Signed-off-by: Saeed Mahameed <saeedm@nvidia.com>