lag_mode describes where the responsibility for LAG table placement lies:
SW or FW. The bus module determines whether LAG is supported, can configure
it if it is, and knows what (if any) configuration has been applied.
Therefore add a bus callback to determine the configured LAG mode. Also add
to core an API to query it.

The LAG mode is for now kept at the default value of 0 for FW-managed. The
code to actually toggle it will be added later.
Signed-off-by: Petr Machata <petrm@nvidia.com>
Reviewed-by: Ido Schimmel <idosch@nvidia.com>
Signed-off-by: David S. Miller <davem@davemloft.net>