mlxsw: spectrum_qdisc: Unify graft validation

Qdisc graft operations have so far been reported at PRIO, ETS and RED, with
RED events ignored, because RED was not considered a classful qdisc. A
following patch will make mlxsw recognize RED and TBF as classful qdiscs,
and thus it is necessary to validate grafting at these qdiscs as well.
Rename the existing graft validator to make it clear that it is a generic
function, and invoke for RED and TBF graft events as well. Drop the
unnecessary PRIO helper and invoke the graft validator directly for PRIO as
well.

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>