uboot-imx

virtio: net: Support non-legacy device

For v1.0 compliant device, it always assumes the member 'num_buffers'
exists in the struct virtio_net_hdr while the legacy driver only
presented 'num_buffers' when VIRTIO_NET_F_MRG_RXBUF was negotiated.
Without that feature the structure was 2 bytes shorter.

Update the driver to support the non-legacy device.

Signed-off-by: Bin Meng <bmeng.cn@gmail.com>
Reviewed-by: Simon Glass <sjg@chromium.org>