mlxsw: spectrum_buffers: Change initialization of PG 9

As explained in commit ff6551ec ("mlxsw: spectrum: Correctly
configure headroom size") control packets are directed to priority group
buffer 9 (PG9) in the ports' headroom buffers.

Since we don't want to drop control packets in case they can't be
admitted to the switch's shared buffer we bind PG9 to a different
ingress pool from the one used by all other PGs.

Unlike other PGs, we currently don't expose the binding between PG9 to a
pool and leave it fixed.

Signed-off-by: Jiri Pirko <jiri@mellanox.com>
Signed-off-by: Ido Schimmel <idosch@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>