mlxsw: spectrum_buffers: Remove eg pool 3 default init and CPU port TC binding to it

Since there is no congestion control for CPU port traffic, we can change
the CPU port TC binding to pool 0 with min_buff and max_buff zeroed.
Remove initialization for pool egress pool 3 since it is no longer used
by dafault.

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