net/mlx5e: ethtool, Fix restriction of autoneg with 56G

Prior to this patch, configuring speed to 50G with autoneg off over
devices supporting 50G per lane failed.
Support for 50G per lane introduced a new set of link-modes, on which
driver always performed a speed validation as if only legacy link-modes
were configured. Fix driver speed validation to force setting autoneg
over 56G only if in legacy link-mode.

Fixes: 3d7cadae

 ("net/mlx5e: ethtool, Fix analysis of speed setting")
Signed-off-by: Aya Levin <ayal@nvidia.com>
Reviewed-by: Eran Ben Elisha <eranbe@nvidia.com>
Signed-off-by: Saeed Mahameed <saeedm@nvidia.com>