virtio_net: Update driver to use ethtool_sprintf

Update the code to replace instances of snprintf and a pointer update with
just calling ethtool_sprintf.

Also replace the char pointer with a u8 pointer to avoid having to recast
the pointer type.

Acked-by: Michael S. Tsirkin <mst@redhat.com>
Acked-by: Jason Wang <jasowang@redhat.com>
Signed-off-by: Alexander Duyck <alexanderduyck@fb.com>
Signed-off-by: David S. Miller <davem@davemloft.net>