net/mlx5e: TC, fix return value check in mlx5e_tc_act_stats_create()

kvzalloc() returns NULL pointer not PTR_ERR() when it fails,
so replace the IS_ERR() check with NULL pointer check.

Fixes: d13674b1

 ("net/mlx5e: TC, map tc action cookie to a hw counter")
Signed-off-by: Yang Yingliang <yangyingliang@huawei.com>
Reviewed-by: Roi Dayan <roid@nvidia.com>
Signed-off-by: Saeed Mahameed <saeedm@nvidia.com>