q_cake: allow changing to diffserv3

q_cake: allow changing to diffserv3

A diffserv3 option (enum value 0) was never sent to the kernel, so it
was not possible to use "tc qdisc change" to select it.

This also meant that were also relying on the kernel's default being
diffserv3 when adding. If the default were to change, we wouldn't have
been able to request diffserv3 explicitly.

Signed-off-by: Kevin Bracey <kevin@bracey.fi>
Acked-by: Toke Høiland-Jørgensen <toke@toke.dk>
Signed-off-by: Stephen Hemminger <stephen@networkplumber.org>