test/pmd_perf: fix crash with multiple devices

test/pmd_perf: fix crash with multiple devices

Error can be reproduce if we run pmd_perf_autotest with more then
one device in such way:
RTE>>set_rxtx_sc poll_before_xmit
RTE>>pmd_perf_autotest

if first burst was value less than MAX_PKT_BURST in the end we overwrite
pkts_burst table for rx which was supposed for another interface.

Fixes: 002ade70e933 ("app/test: measure cycles per packet in Rx/Tx")
Cc: stable@dpdk.org
Signed-off-by: Daniel Mrzyglod <danielx.t.mrzyglod@intel.com>
Acked-by: Michal Jastrzebski <michalx.k.jastrzebski@intel.com>