mlxsw: spectrum_buffers: Forbid changing threshold type of first egress pool

Multicast packets have three egress quotas:
* Per egress port
* Per egress port and traffic class
* Per switch priority

The limits on the switch priority are not exposed to the user and
specified as dynamic threshold on the first egress pool.

Forbid changing the threshold type of the first egress pool so that
these limits are always valid.

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>