r6040: enable transmit time stamping.

This patch enables software (and phy device) transmit time stamping
for the RDC R6040 Fast Ethernet MAC. Compile tested only.

Cc: Florian Fainelli <florian@openwrt.org>
Signed-off-by: Richard Cochran <richard.cochran@omicron.at>
Signed-off-by: David S. Miller <davem@conan.davemloft.net>