net: bcmgenet: add skb_tx_timestamp call

The BCMGENET driver was not TX timestamping the SKBs it queued for
transmission, do this in bcmgenet_xmit() right before kicking the
Transmit DMA engine.

Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>