igc: Fix ambiguity in the ethtool advertising

The 'ethtool_convert_link_mode_to_legacy_u32' method does not allow us to
advertise 2500M speed support and TP (twisted pair) properly. Convert to
'ethtool_link_ksettings_test_link_mode' to advertise supported speed and
eliminate ambiguity.

Fixes: 8c5ad0da

 ("igc: Add ethtool support")
Suggested-by: Dima Ruinskiy <dima.ruinskiy@intel.com>
Suggested-by: Vitaly Lifshits <vitaly.lifshits@intel.com>
Signed-off-by: Sasha Neftin <sasha.neftin@intel.com>
Tested-by: Naama Meir <naamax.meir@linux.intel.com>
Signed-off-by: Jacob Keller <jacob.e.keller@intel.com>
Link: https://lore.kernel.org/r/20231019203641.3661960-1-jacob.e.keller@intel.com


Signed-off-by: Jakub Kicinski <kuba@kernel.org>