xen-netfront: fix oops when disconnected from backend

xennet_disconnect_backend() was not correctly iterating over all the
queues.

Signed-off-by: David Vrabel <david.vrabel@citrix.com>
Reviewed-by: Wei Liu <wei.liu2@citrix.com>
Signed-off-by: David S. Miller <davem@davemloft.net>