igc: Refactor the igc_power_down_link()

Currently the implementation of igc_power_down_link()
method was just calling igc_power_down_phy_copper_base()
method.
We can just call igc_power_down_phy_copper_base()
method directly.

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>