mlxsw: spectrum: qdiscs: Clean qdisc statistics structs

Clean RED offloaded stats and make them more generic by breaking the
generic qdisc stats to a struct of their own.

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