sfc: add support for skb->xmit_more

Don't ring the doorbell, and don't do PIO.  This will also prevent
 TX Push, because there will be more than one buffer waiting when
 the doorbell is rung.

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