mlxsw: spectrum: Set SW LAG mode on Spectrum>1

On Spectrum-2, Spectrum-3 and Spectrum-4 machines, request SW
responsibility for placement of the LAG table.

On Spectrum-1, some FW versions claim to support lag_mode field despite
quietly ignoring any settings made to that field. Thus refrain from
attempting to configure lag_mode on those systems at all.

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