vtnet: don't provide VIRTIO_NET_HDR_F_DATA_VALID

According to section 5.1.6.2.1 of version 1.3 of the virtio
specification, the driver MUST NOT set VIRTIO_NET_HDR_F_DATA_VALID in
the flags. So don't do that.

Reviewed by: Timo Völker
MFC after: 3 days
Differential Revision: https://reviews.freebsd.org/D53650