test/pmd_perf: fix the way to drain the port

test/pmd_perf: fix the way to drain the port

If the port has received less than ``pkt_per_port`` packets (for
example, the port has missed some packets), the test is in an infinite
loop.

Instead of expecting a number of packet to receive, let the port to be
drained by itself. If no more packets are received, the test can
continue.

Fixes: 002ade70e933 ("app/test: measure cycles per packet in Rx/Tx")
Cc: stable@dpdk.org
Signed-off-by: Julien Meunier <julien.meunier@nokia.com>
Reviewed-by: Ferruh Yigit <ferruh.yigit@intel.com>