ixgbe: should not use HW_CSUM, should use IP* flags

as mentioned by Herbert, our hardware supports IP offloads, not full
checksum offloads for any protocol in existence (even though the
hardware just provides generic csum support over any range of bytes)

Signed-off-by: Jesse Brandeburg <jesse.brandeburg@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>
Signed-off-by: Jeff Garzik <jgarzik@redhat.com>