sfc: on 8000 series use TX queues for TX timestamps

For this we create and use one or more new TX queues on the PTP channel,
 and enable sync events for it.
Based on a patch by Martin Habets <mhabets@solarflare.com>.

Signed-off-by: Edward Cree <ecree@solarflare.com>
Signed-off-by: David S. Miller <davem@davemloft.net>