ixgbe: fix rx csum return status misinterpretation

the driver was misinterpreting rx_csum return value in the descriptor
so occassionally we would indicate an rx_csum error in our stats
when there was none.  This would have no effect on traffic because
we would just hand the packet to the stack anyway without the offload
flag set, but would increase CPU for those packets that needed a recompute.

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>