net/mlx5: ethtool, Allow legacy link-modes configuration via non-extended ptys

Allow configuration of legacy link-modes even when extended link-modes
are supported. This requires reading of legacy advertisement even when
extended link-modes are supported. Since legacy and extended
advertisement are mutually excluded, wait for empty reply from extended
advertisement before reading legacy advertisement.

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>