mlxsw: spectrum: Support general qdisc stats

Add support for ndo_setup_tc with enum tc_setup_type value of
TC_SETUP_QDISC_STATS. This call updates the generic qdisc stats from the
cache if the handle ID that is asked for matching the root qdisc ID and
fails otherwise.
Currently doesn't support qlen and rqueues.

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