mlxsw: spectrum_qdisc: Clean stats recursively when priomap changes

On Spectrum, there are no per-TC TX counters. Instead, mlxsw uses per-prio
counters and aggregates them according to the priomap. Therefore when
priomap changes, the counter base values need to be reset to reflect the
change. Previously, this was only done for the sole child qdisc, but a
following patch makes RED and TBF classful. Thus apply the request to the
whole sub-tree.

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>