net/mlx5: Add stub for mlx5_eswitch_mode

Return MLX5_ESWITCH_NONE when CONFIG_MLX5_ESWITCH
is not selected.

Signed-off-by: Maor Gottlieb <maorg@mellanox.com>
Reviewed-by: Mark Bloch <markb@mellanox.com>
Signed-off-by: Saeed Mahameed <saeedm@mellanox.com>