net/mlx5e: kTLS, Use _safe() iterator in mlx5e_tls_priv_tx_list_cleanup()

Use the list_for_each_entry_safe() macro to prevent dereferencing "obj"
after it has been freed.

Fixes: c4dfe704 ("net/mlx5e: kTLS, Recycle objects of device-offloaded TLS TX connections")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Reviewed-by: Tariq Toukan <tariqt@nvidia.com>
Signed-off-by: Saeed Mahameed <saeedm@nvidia.com>