igc: Remove no need declaration of the igc_power_down_link

We want to avoid forward-declarations of function if possible.
Rearrange the igc_power_down_link function implementation.

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>