mlxsw: spectrum_buffers: Forbid configuration of multicast pool

mlxsw: spectrum_buffers: Forbid configuration of multicast pool

Commit 6031b39c4331 ("mlxsw: spectrum_buffers: Configure MC pool") added
a dedicated pool for multicast traffic. The pool is visible to the user
so that it would be possible to monitor its occupancy, but its
configuration should be forbidden in order to maintain its intended
operation.

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