net/mlx5: Split the activate lag function into two routines

Split the activate lag function in order to be symmetric with
the deactivate lag function.

Signed-off-by: Roi Dayan <roid@mellanox.com>
Signed-off-by: Shahar Klein <shahark@mellanox.com>
Reviewed-by: Aviv Heller <avivh@mellanox.com>
Signed-off-by: Saeed Mahameed <saeedm@mellanox.com>