net/mlx5: Rename mlx5_lag_is_bonded() to __mlx5_lag_is_active()

The new name better represents the function's aim, and sets a precedent
for a '__' prefix for internal, non-locked versions of LAG APIs.

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