mlxsw: spectrum: Add note about the PTP shaper

Add note about disabling the PTP shaper when calling to
mlxsw_sp_port_ets_maxrate_set().

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