net/mlx5e: CT: Correctly get flow rule

The correct way is to us the flow_cls_offload_flow_rule() wrapper
instead of f->rule directly.

Fixes: 4c3844d9 ("net/mlx5e: CT: Introduce connection tracking")
Signed-off-by: Roi Dayan <roid@mellanox.com>
Reviewed-by: Oz Shlomo <ozsh@mellanox.com>
Signed-off-by: Saeed Mahameed <saeedm@mellanox.com>