net: sched: don't set extack message in case the qdisc will be created

If the qdisc is not found here, it is going to be created. Therefore,
this is not an error path. Remove the extack message set and don't
confuse user with error message in case the qdisc was created
successfully.

Fixes: 09215598 ("net: sched: sch_api: handle generic qdisc errors")
Signed-off-by: Jiri Pirko <jiri@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>