virtio_net: remove unused parameter to send_command

All the code passes NULL for the last sg list (in).
Simplify by just removing it.

Signed-off-by: Stephen Hemminger <stephen@networkplumber.org>
Acked-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>