amd-xgbe: Add support for the skb->xmit_more flag

Add support to delay telling the hardware about data that is ready to
be transmitted if the skb->xmit_more flag is set.

Signed-off-by: Tom Lendacky <thomas.lendacky@amd.com>
Signed-off-by: David S. Miller <davem@davemloft.net>