sfc: handle TX timestamps in the normal data path

Before this work, TX timestamping is done by sending each SKB to the MC.
On the 8000 series (Medford1) we have high speed timestamping via the
 MAC, which means we can use normal TX queues for this without a
 significant drop in bandwidth.  On the X2000 series (Medford2) support
 for transmitting via the MC is removed, so the new way must be used.

This patch enables timestamping on a TX queue, if requested.
It also enhances TX event handling to process the extra completion events,
 and puts the time in the SKB.

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