virtio-net: fix mismatch of getting tx-frames

Since virtio-net allows switching napi_tx for per txq, we have to
get the specific txq's result now.

Fixes: 394bd877 ("virtio_net: support per queue interrupt coalesce command")
Cc: Gavin Li <gavinl@nvidia.com>
Signed-off-by: Heng Qi <hengqi@linux.alibaba.com>
Acked-by: Jason Wang <jasowang@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>