forcedeth: advertise transmit time stamping

This driver now offers software transmit time stamping, so it should
advertise that fact via ethtool. Compile tested only.

Signed-off-by: Richard Cochran <richardcochran@gmail.com>

Cc: Willem de Bruijn <willemb@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>