r8169: enable transmit time stamping.

This patch has been tested on a machine with the Realtek
RTL8101E/RTL8102E PCI Express Fast Ethernet controller (rev 05).

Cc: Realtek linux nic maintainers <nic_swsd@realtek.com>
Cc: Francois Romieu <romieu@fr.zoreil.com>
Signed-off-by: Richard Cochran <richardcochran@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>