app/testpmd: fix stats error message

app/testpmd: fix stats error message

There are coverity defects related "Argument cannot be negative"

This patch fixes them by passing '-ret' to the function strerror() when
ret is negative.

Coverity issue: 349913, 358437, 358449, 358450
Fixes: da328f7f115a ("ethdev: change xstats reset function to return int")
Fixes: 9eb974221f44 ("app/testpmd: fix statistics after reset")
Cc: stable@dpdk.org
Signed-off-by: Wei Hu (Xavier) <xavier.huwei@huawei.com>
Reviewed-by: Ferruh Yigit <ferruh.yigit@intel.com>