netdev-dpdk: Do not flush tx queue which is shared among CPUs since it is always...

netdev-dpdk: Do not flush tx queue which is shared among CPUs since it is always flushed

When tx queue is shared among CPUS,the pkts always be flush
in 'netdev_dpdk_eth_send'. So it is unnecessarily for flushing
in netdev_dpdk_rxq_recv Otherwise tx will be accessed without
locking.

Signed-off-by: Wei li <liw@dtdream.com>
Acked-by: Daniele Di Proietto <diproiettod@vmware.com>
Acked-by: Pravin B Shelar <pshelar@nicira.com>