mlxsw: spectrum_qdisc: Destroy children in mlxsw_sp_qdisc_destroy()

Currently ETS and PRIO are the only offloaded classful qdiscs. Since they
are both similar, their destroy handler is the same, and it handles
children destruction itself. But now it is possible to do it generically
for any classful qdisc. Therefore promote the recursive destruction from
the ETS handler to mlxsw_sp_qdisc_destroy(), so that RED and TBF pick it up
in follow-up patches.

Signed-off-by: Petr Machata <petrm@nvidia.com>
Signed-off-by: Ido Schimmel <idosch@nvidia.com>
Signed-off-by: David S. Miller <davem@davemloft.net>