net/mlx5e: ethtool, Avoid setting speed to 56GBASE when autoneg off

Setting speed to 56GBASE is allowed only with auto-negotiation enabled.

This patch prevent setting speed to 56GBASE when auto-negotiation disabled.

Fixes: f62b8bb8

 ("net/mlx5: Extend mlx5_core to support ConnectX-4 Ethernet functionality")
Signed-off-by: Mohamad Heib <mohamadh@mellanox.com>
Signed-off-by: Saeed Mahameed <saeedm@mellanox.com>