mlxsw: spectrum: qdiscs: Create a generic destroy function

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 <[email protected]>
Reviewed-by: Yuval Mintz <[email protected]>
Signed-off-by: Jiri Pirko <[email protected]>
Signed-off-by: David S. Miller <[email protected]>