dnet: enable transmit time stamping.

This patch enables software (and phy device) transmit time stamping
in the "Dave ethernet interface." Compile tested only.

Cc: Ilya Yanok <yanok@emcraft.com>
Signed-off-by: Richard Cochran <richard.cochran@omicron.at>
Signed-off-by: David S. Miller <davem@conan.davemloft.net>