net/mlx5: Expose mlx5e_link_mode

The mlx5e_link_mode enumeration will also be used in mlx5_ib for RoCE.
This patch moves the enumeration to the mlx5 driver port header file.

Signed-off-by: Noa Osherovich <noaos@mellanox.com>
Signed-off-by: Eran Ben Elisha <eranbe@mellanox.com>
Signed-off-by: Saeed Mahameed <saeedm@mellanox.com>
Signed-off-by: Leon Romanovsky <leon@kernel.org>