mlxsw: spectrum_buffers: Forbid changing multicast TCs' attributes

Commit e83c045e ("mlxsw: spectrum_buffers: Configure MC pool")
configured the threshold of the multicast TCs as infinite so that the
admission of multicast packets is only depended on per-switch priority
threshold.

Forbid the user from changing the thresholds of these multicast TCs and
their binding to a different pool.

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