net: e1000e: support txtd update delay via xmit_more

Don't update Tx tail descriptor if queue hasn't been stopped
and we know at least one more skb will be sent right away.

Signed-off-by: Florian Westphal <fw@strlen.de>
Tested-by: Aaron Brown <aaron.f.brown@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>