mlxsw: spectrum: Set port's shared buffer size to 0

In addition to the priority group (PG) buffers in the headroom, the
device enables the allocation of headroom shared buffer, which can
be shared between different PGs.

However, we are not going to use the headroom shared buffer and instead
allow the user to use its size for PGs or the switch's shared buffer.

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