smsc9420: enable transmit time stamping.

This patch enables software (and phy device) transmit time stamping
for the smsc9420. Compile tested only.

Cc: Steve Glendinning <steve.glendinning@smsc.com>
Signed-off-by: Richard Cochran <richard.cochran@omicron.at>
Signed-off-by: David S. Miller <davem@conan.davemloft.net>