app/testpmd: fix config crash
app/testpmd: fix config crash

A chained topology must always have an increment of 1.
Here, it was 2 if ports number is even.

Signed-off-by: Damien Millescamps <4b72284df888c49ff7aaa7196e02c39d02a0641e@6wind.com>
Acked-by: Ivan Boule <8408c61bea57ac94dd0d4dac44b115be1b288952@6wind.com>
Acked-by: Thomas Monjalon <ee2fa8b2681c737f775687e475dcbf2028a412f1@6wind.com>

john-mcnamara-intel/dpdk,msune/dpdk,venkynv/dpdk-mirror,msune/dpdk,tsphillips/dpdk-fork,tsphillips/dpdk-fork,tsphillips/dpdk-fork,john-mcnamara-intel/dpdk,mixja/dpdk,mixja/dpdk,mixja/dpdk,msune/dpdk,venkynv/dpdk-mirror,venkynv/dpdk-mirror,tsphillips/dpdk-fork,venkynv/dpdk-mirror,msune/dpdk,john-mcnamara-intel/dpdk,john-mcnamara-intel/dpdk,mixja/dpdk