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>