virtio_net: use LE accessors for speed/duplex

Speed and duplex config fields depend on VIRTIO_NET_F_SPEED_DUPLEX
which being 63>31 depends on VIRTIO_F_VERSION_1.

Accordingly, use LE accessors for these fields.

Reported-by: Cornelia Huck <cohuck@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>