mlxsw: spectrum_acl: assign vchunk->chunk by the newly created chunk

Make the vchunk->chunk contain pointer of a new chunk we migrate to.
In case of a rollback, it contains the original chunk.

Signed-off-by: Jiri Pirko <jiri@mellanox.com>
Signed-off-by: Ido Schimmel <idosch@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>