mlxsw: pci: Permit toggling LAG mode

Add to struct mlxsw_config_profile a field lag_mode_prefer_sw for the
driver to indicate that SW LAG mode should be configured if possible. Add
to the PCI module code to set lag_mode as appropriate.

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