igc: Remove PCIe Control register

GCR (PCIe Control) register not in use and should be removed
This patch clean up this register

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>