net/mlx5: Bridge, support replacing existing FDB entry

The SWITCHDEV_FDB_ADD_TO_DEVICE is used for both adding new and replacing
existing entry. Implement support for replacing existing FDB entries in
mlx5 offload code.

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