ethtool: Add a common function for drivers with transmit time stamping.

Currently, most drivers do not support transmit SO_TIMESTAMPING. For those
that do support it, there is one appropriate response to the get_ts_info
query. This patch adds a common function providing this response.

Signed-off-by: Richard Cochran <richardcochran@gmail.com>
Reviewed-by: Ben Hutchings <bhutchings@solarflare.com>
Signed-off-by: David S. Miller <davem@davemloft.net>