net/mlx5: Lag, don't update lag if lag isn't supported

In NICs that don't support LAG, the LAG control structure won't be
allocated. If it wasn't allocated it means LAG doesn't exists and can be
skipped.

Fixes: cac1eb2c

 ("net/mlx5: Lag, properly lock eswitch if needed")
Signed-off-by: Mark Bloch <mbloch@nvidia.com>
Reviewed-by: Maor Gottlieb <maorg@nvidia.com>
Reviewed-by: Leon Romanovsky <leonro@nvidia.com>
Signed-off-by: Saeed Mahameed <saeedm@nvidia.com>