igc: Remove unused registers

Tx data FIFO Head/Tail, Saved and Packet Count registers
not applicable for i225 LAN controller.
This patch comes to clean up these registers.

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>