examples/dma: fix Tx drop statistics

examples/dma: fix Tx drop statistics

The Tx drop statistic was designed to be collected by
rte_eth_dev_tx_buffer mechanism, but the application uses
rte_eth_tx_burst to send packets and this lead the Tx drop statistic
was not collected.

This patch removes rte_eth_dev_tx_buffer mechanism to fix the problem.

Fixes: 632bcd9b5d4f ("examples/ioat: print statistics")
Cc: stable@dpdk.org
Signed-off-by: Chengwen Feng <fengchengwen@huawei.com>
Acked-by: Bruce Richardson <bruce.richardson@intel.com>
Acked-by: Kevin Laatz <kevin.laatz@intel.com>