usbnet: Fix tx_packets stat for FLAG_MULTI_FRAME drivers

Currently the usbnet core does not update the tx_packets statistic for
drivers with FLAG_MULTI_PACKET and there is no hook in the TX
completion path where they could do this.

cdc_ncm and dependent drivers are bumping tx_packets stat on the
transmit path while asix and sr9800 aren't updating it at all.

Add a packet count in struct skb_data so these drivers can fill it
in, initialise it to 1 for other drivers, and add the packet count
to the tx_packets statistic on completion.

Signed-off-by: Ben Hutchings <ben.hutchings@codethink.co.uk>
Tested-by: Bjørn Mork <bjorn@mork.no>
Signed-off-by: David S. Miller <davem@davemloft.net>
diff --git a/drivers/net/usb/usbnet.c b/drivers/net/usb/usbnet.c
index 449835f..0f3ff28 100644
--- a/drivers/net/usb/usbnet.c
+++ b/drivers/net/usb/usbnet.c
@@ -1188,8 +1188,7 @@
 	struct usbnet		*dev = entry->dev;
 
 	if (urb->status == 0) {
-		if (!(dev->driver_info->flags & FLAG_MULTI_PACKET))
-			dev->net->stats.tx_packets++;
+		dev->net->stats.tx_packets += entry->packets;
 		dev->net->stats.tx_bytes += entry->length;
 	} else {
 		dev->net->stats.tx_errors++;
@@ -1348,6 +1347,8 @@
 			urb->transfer_flags |= URB_ZERO_PACKET;
 	}
 	entry->length = urb->transfer_buffer_length = length;
+	if (!(info->flags & FLAG_MULTI_PACKET))
+		usbnet_set_skb_tx_stats(skb, 1);
 
 	spin_lock_irqsave(&dev->txq.lock, flags);
 	retval = usb_autopm_get_interface_async(dev->intf);