igc: Remove unused CTRL_EXT_LINK_MODE_MASK

We support only copper mode
Not applicable for i225 parts

Signed-off-by: Sasha Neftin <sasha.neftin@intel.com>
Tested-by: Aaron Brown <aaron.f.brown@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>