net/mlx5: Lag, Make mlx5_lag_is_multipath() be static inline

Fix "no previous prototype" W=1 warnings when CONFIG_MLX5_CORE_EN is not set:

  drivers/net/ethernet/mellanox/mlx5/core/lag_mp.h:34:6: error: no previous prototype for ‘mlx5_lag_is_multipath’ [-Werror=missing-prototypes]
     34 | bool mlx5_lag_is_multipath(struct mlx5_core_dev *dev) { return false; }
        |      ^~~~~~~~~~~~~~~~~~~~~

Fixes: 14fe2471 ("net/mlx5: Lag, change multipath and bonding to be mutually exclusive")
Signed-off-by: Maor Dickman <maord@nvidia.com>