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>

drivers/net/ethernet/mellanox/mlx5/core/lag/mp.h

@@ -31,7 +31,7 @@ bool mlx5_lag_is_multipath(struct mlx5_core_dev *dev);
 static inline void mlx5_lag_mp_reset(struct mlx5_lag *ldev) {};
 static inline int mlx5_lag_mp_init(struct mlx5_lag *ldev) { return 0; }
 static inline void mlx5_lag_mp_cleanup(struct mlx5_lag *ldev) {}
-bool mlx5_lag_is_multipath(struct mlx5_core_dev *dev) { return false; }
+static inline bool mlx5_lag_is_multipath(struct mlx5_core_dev *dev) { return false; }
 
 #endif /* CONFIG_MLX5_ESWITCH */
 #endif /* __MLX5_LAG_MP_H__ */