app/flow-perf: fix parsing of invalid option

app/flow-perf: fix parsing of invalid option

Currently, if an invalid parameter is passed to the application
it will cause a crash due to missing default in options.

For example:
        ./dpdk-test-flow-perf -a 01:00.0 -- --invalid

This adds missing default for options, and prints the
invalid option.

Fixes: 3344cf2e3001 ("app/flow-perf: add flow performance skeleton")
Cc: stable@dpdk.org
Signed-off-by: Raslan Darawsheh <rasland@nvidia.com>
Acked-by: Wisam Jaddo <wisamm@nvidia.com>