igb: cleanup "todo" code found in igb_ethtool.c

This patch moves some defines into the e1000_regs.h file since this is the
correct place for register defines and not inside of igb_ethtool.c

Signed-off-by: Alexander Duyck <alexander.h.duyck@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>