igc: Remove no need declaration of the igc_sw_init

We want to avoid forward-declarations of function if possible.
Rearrange the igc_sw_init function implementation.
Signed-off-by: NSasha Neftin <sasha.neftin@intel.com>
Tested-by: NAaron Brown <aaron.f.brown@intel.com>
Signed-off-by: NJeff Kirsher <jeffrey.t.kirsher@intel.com>