examples/ipsec-secgw: fix use of ethdev internal struct

examples/ipsec-secgw: fix use of ethdev internal struct

Modify ipsec-secgw example app to use rte_eth_dev_info_get instead of
rte_eth_dev.
Apps should not be using internal DPDK data structures directly.

Fixes: a4677f78368b ("examples/ipsec-secgw: add target queues in flow actions")
Cc: stable@dpdk.org
Signed-off-by: Marcin Zapolski <marcinx.a.zapolski@intel.com>
Reviewed-by: Bruce Richardson <bruce.richardson@intel.com>
Acked-by: Nelio Laranjeiro <nelio.laranjeiro@6wind.com>