tg3: Add skb->xmit_more support

Ring TX doorbell only if xmit_more is not set or the queue is stopped.

Suggested-by: Daniel Borkmann <dborkman@redhat.com>
Signed-off-by: Prashant Sreedharan <prashant@broadcom.com>
Signed-off-by: Michael Chan <mchan@broadcom.com>
Signed-off-by: David S. Miller <davem@davemloft.net>