mlxsw: spectrum_buffers: Prevent changing CPU port's configuration

Next patch is going to register the CPU port with devlink, but only so
that the CPU port's shared buffer configuration and occupancy could be
queried.

Prevent changing CPU port's shared buffer threshold and binding
configuration.

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