mlxsw: spectrum_qdisc: Support offloading of ETS Qdisc

Handle TC_SETUP_QDISC_ETS, add a new ops structure for the ETS Qdisc.
Invoke the extended prio handlers implemented in the previous patch. For
stats ops, invoke directly the prio callbacks, which are not sensitive to
differences between PRIO and ETS.

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