net/bonding: fix selection logic

Arrays agg_count and agg_bandwidth should be indexed by slave_id not by
aggregator port_id.

The new_agg_id should be chosen as slave_id from slaves table in
different selection modes.

Fixes: 6d72657ce379 ("net/bonding: add other aggregator modes")
Cc: stable@dpdk.org
Signed-off-by: Krzysztof Kanas <kkanas@marvell.com>
Acked-by: Chas Williams <chas3@att.com>
Acked-by: Ferruh Yigit <ferruh.yigit@intel.com>