virtio-net: a tiny comment update

Update a comment because virtio-net now supports both
VIRTIO_NET_F_NOTF_COAL and VIRTIO_NET_F_VQ_NOTF_COAL.

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>