drivers: net: aurora: use netdev_xmit_more helper

This is the last driver using always-0 skb->xmit_more.
Switch it to netdev_xmit_more and remove the now unused xmit_more flag
from sk_buff.

Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: David S. Miller <davem@davemloft.net>