test: avoid hang if queues are full and Tx fails

test: avoid hang if queues are full and Tx fails

Current pmd_perf_autotest() in continuous mode tries
to enqueue MAX_TRAFFIC_BURST completely before starting
the test. Some drivers cannot accept complete
MAX_TRAFFIC_BURST even though rx+tx desc count can fit it.
This patch changes behaviour to stop enqueuing after few
retries.

Fixes: 002ade70e933 ("app/test: measure cycles per packet in Rx/Tx")
Cc: stable@dpdk.org
Signed-off-by: Rakesh Kudurumalla <rkudurumalla@marvell.com>