net/mlx5: Lag, Use flag to check for shared FDB mode

It's redundant and incorrect to check lag is also sriov mode.

Signed-off-by: Mark Bloch <mbloch@nvidia.com>
Reviewed-by: Roi Dayan <roid@nvidia.com>
Signed-off-by: Saeed Mahameed <saeedm@nvidia.com>