net/mlx5e: TC, Destroy nic flow counter if exists

Counter is only added if counter flag exists.
So check the counter fag exists for deleting the counter.
This is the same as in add/del fdb flow.

Signed-off-by: Roi Dayan <roid@nvidia.com>
Reviewed-by: Vlad Buslov <vladbu@nvidia.com>
Signed-off-by: Saeed Mahameed <saeedm@nvidia.com>