app/crypto-perf: fix CSV output

When using CSV, the output of the throughput performance numbers
was not correct, representing decimal numbers incorrectly.

Fixes: f8be1786b1b8 ("app/crypto-perf: introduce performance test application")
Cc: stable@dpdk.org
Signed-off-by: Pablo de Lara <pablo.de.lara.guarch@intel.com>