drivers: net: sfc: use netdev_xmit_more helper

skb->xmit_more hint is now always 0, this switches the sfc driver to
use the netdev_xmit_more helper instead.

Cc: Solarflare linux maintainers <linux-net-drivers@solarflare.com>
Cc: Edward Cree <ecree@solarflare.com>
Cc: Bert Kenward <bkenward@solarflare.com>
Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: David S. Miller <davem@davemloft.net>