sfc: MAC TX timestamp handling on the 8000 series

TX timestamps on 8000 series are supplied from the MAC. This timestamp is
 only 48 bits long. The high order bits from the last time sync event are
 used for the top 16 bits.

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>