mlxsw: spectrum_buffers: Get max_buff defaults into limits exposed to user

Although the device supports max_buff magic values 0 and 0xff, these are
not exposed to the user via devlink.
Therefore, adjust the default values to be within configurable range.

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