net/mlx5e: Move port speed code from en_ethtool.c to en/port.c

Move four below functions from en_ethtool.c to en/port.c. These
functions are used by both en_ethtool.c and en_main.c. Future code
can use these functions without ethtool link mode dependency.
  u32 mlx5e_port_ptys2speed(u32 eth_proto_oper);
  int mlx5e_port_linkspeed(struct mlx5_core_dev *mdev, u32 *speed);
  int mlx5e_port_max_linkspeed(struct mlx5_core_dev *mdev, u32 *speed);
  u32 mlx5e_port_speed2linkmodes(u32 speed);

Delete the speed field from table mlx5e_build_ptys2ethtool_map. This
table only keeps the mapping between the mlx5e link mode and
ethtool link mode. Add new table mlx5e_link_speed for translation
from mlx5e link mode to actual speed.

Signed-off-by: Huy Nguyen <huyn@mellanox.com>
Reviewed-by: Parav Pandit <parav@mellanox.com>
Signed-off-by: Saeed Mahameed <saeedm@mellanox.com>