mlxsw: spectrum: qdiscs: prio: Handle graft command

Handle graft command for an offloaded sch_prio.
Grafting a qdisc to any place other than under its original parent is not
supported by mlxsw and will cause the grafted qdisc to stop being
offloaded.

Signed-off-by: Nogah Frankel <nogahf@mellanox.com>
Reviewed-by: Yuval Mintz <yuvalm@mellanox.com>
Signed-off-by: Jiri Pirko <jiri@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>