igb: cleanup incorrect comment and set IP Checksum Enable

IP Checksum enable doesn't need packet split in order to function.  It only
requires the use of advanced descriptors which the current igb driver does.
So we can enable it always without any issues.

Signed-off-by: Alexander Duyck <alexander.h.duyck@intel.com>
Acked-by: Peter P Waskiewicz Jr <peter.p.waskiewicz.jr@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>