mlxsw: spectrum_buffers: Keep shared buffer size in mlxsw_sp_sb

Entities of infinite size will be reported as if they had the maximum
size allowed by the chip. To that end, keep track of maximum shared
buffer size in mlxsw_sp->sb.

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