usbnet: add timestamping support

In order to make USB-to-Ethernet-adapters (depending on usbnet) support
timestamping, the "skb_defer_rx_timestamp" and "skb_tx_timestamp" function
calls are added.

Signed-off-by: Michael Riesch <michael@riesch.at>
Signed-off-by: David S. Miller <davem@davemloft.net>