virtio_net: replace vq2rxq with vq2txq where appropriate

While not harmful, using vq2rxq where it's always sq appears odd.
Replace it with the more appropriate vq2txq for clarity and correctness.

Fixes: 89f86675 ("virtio_net: xsk: tx: support xmit xsk buffer")
Signed-off-by: Koichiro Den <koichiro.den@canonical.com>
Acked-by: Jason Wang <jasowang@redhat.com>
Reviewed-by: Xuan Zhuo <xuanzhuo@linux.alibaba.com>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>