rtl8139: cleanup FCS calculation

clean out ifdef's around ethernet checksum calculation

Signed-off-by: Benjamin Poirier <benjamin.poirier@gmail.com>
Acked-by: Igor V. Kovalenko <igor.v.kovalenko@gmail.com>
Cc: Jason Wang <jasowang@redhat.com>
Cc: Michael S. Tsirkin <mst@redhat.com>
Cc: Blue Swirl <blauwirbel@gmail.com>
Signed-off-by: Blue Swirl <blauwirbel@gmail.com>
