igb: reenable CRC stripping in hardware

We can remove a clunky workaround for not having the hardware
strip the CRC. 82575 silicon as well as the older PCI Express
e1000e hardware all work OK in this respect.

Signed-off-by: Auke Kok <auke-jan.h.kok@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>
Signed-off-by: Jeff Garzik <jgarzik@redhat.com>