net/mlx5e: Use hard_mtu as part of the mlx5e_priv struct

The mtu extra space that kept for the HW is specific for each link type,
and it is different in mlx5e and mlx5i modules.
Now it is kept in the priv structures, set by the mlx5e/mlx5i driver
accordingly.

Signed-off-by: Erez Shitrit <erezsh@mellanox.com>
Signed-off-by: Saeed Mahameed <saeedm@mellanox.com>