net/mlx5: ethtool, Fix type analysis of advertised link-mode

Ethtool option set_link_ksettings allows setting of legacy link-modes
or extended link-modes. Refine the decision of which type of link-modes
is set.

Fixes: 6a897372

 ("net/mlx5: ethtool, Add ethtool support for 50Gbps per lane link modes")
Signed-off-by: Aya Levin <ayal@mellanox.com>
Signed-off-by: Saeed Mahameed <saeedm@mellanox.com>