net: usbnet: Update net_device stats on tx and rx success

Currently the driver does not update usbnet->net_device->stats
tx and rx success case statistics in tx_complete and
usbnet_skb_return. As a result, the net drivers having their own
definiton for ndo_get_stats are not able to update success case
stats leading to incomplete info in ifconfig command output.
Fix this by also updating usbnet->net_device->stats on tx and rx
success.

Change-Id: I881343644c92d5031300edfa7340a503ab4cbf55
Signed-off-by: Ajay Agarwal <ajaya@codeaurora.org>