gve: Correctly report software timestamping capabilities

gve has supported software timestamp generation since its inception,
but has not advertised that support via ethtool. This patch correctly
advertises that support.

Signed-off-by: John Fraker <jfraker@google.com>
Reviewed-by: Harshitha Ramamurthy <hramamurthy@google.com>
Reviewed-by: Willem de Bruijn <willemb@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>