net/mlx5e: Fix use-after-free when reverting termination table

When having multiple dests with termination tables and second one
or afterwards fails the driver reverts usage of term tables but
doesn't reset the assignment in attr->dests[num_vport_dests].termtbl
which case a use-after-free when releasing the rule.
Fix by resetting the assignment of termtbl to null.

Fixes: 10caabda ("net/mlx5e: Use termination table for VLAN push actions")
Signed-off-by: Roi Dayan <roid@nvidia.com>
Reviewed-by: Maor Dickman <maord@nvidia.com>
Signed-off-by: Saeed Mahameed <saeedm@nvidia.com>