mlxsw: spectrum: Add mlxsw_sp_span_ops.buffsize_get for Spectrum-3

The buffer factor on Spectrum-3 is larger than on Spectrum-2. Add a new
callback and use it for mlxsw_sp->span_ops on Spectrum-3.

Signed-off-by: Petr Machata <petrm@mellanox.com>
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>