examples/flow_filtering: check status of getting ethdev info

rte_eth_dev_info_get() return value was changed from void to
int, so this patch modify rte_eth_dev_info_get() usage across
examples/flow_filtering according to its new return type.

Signed-off-by: Ivan Ilchenko <ivan.ilchenko@oktetlabs.ru>
Signed-off-by: Andrew Rybchenko <arybchenko@solarflare.com>
Acked-by: Ori Kam <orika@mellanox.com>