mlxsw: spectrum: qdiscs: Create a generic destroy function

Add a destroy function to the qdiscs ops struct.
Create a generic qdisc destroy function, that clears the qdisc metadata as
well as calling the specific qdisc destroy function.

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>