app/testpmd: fix packet throughput after stats reset

app/testpmd: fix packet throughput after stats reset

Testpmd calculates packet throughput by getting a diff of previous stats
value and current one.

If a stats clear called after previous sample taken, the diff will be
negative and throughput calculation will be wrong.

If current stats value is smaller than previous one, set throughput to
zero.

Fixes: 0e106980301d ("app/testpmd: show throughput in port stats")
Cc: stable@dpdk.org
Signed-off-by: Wei Zhao <wei.zhao1@intel.com>
Reviewed-by: Ferruh Yigit <ferruh.yigit@intel.com>