net/bonding: fix RSS key length

net/bonding: fix RSS key length

Currently the hash_key_size information has not been set. So, apps can
not get the key size from dev_info(), this make some problem.

e.g, in testpmd, the hash_key_size will be checked before configure
or get the hash key:
testpmd> show port 4 rss-hash
dev_info did not provide a valid hash key size
testpmd> show port 4 rss-hash key
dev_info did not provide a valid hash key size
testpmd> port config 4 rss-hash-key ipv4 (hash key)
dev_info did not provide a valid hash key size

In this patch, the meaning of rss_key_len has been modified. It only
indicated the length of the configured hash key before. Therefore,
its value depends on the user's configuration. This seems unreasonable.
And now, it indicates the minimum hash key length required by the
bonded device. Its value will be the shortest hash key among all slave
drivers.

Fixes: 734ce47f71e0 ("bonding: support RSS dynamic configuration")
Cc: stable@dpdk.org
Signed-off-by: Chengchang Tang <tangchengchang@huawei.com>
Signed-off-by: Min Hu (Connor) <humin29@huawei.com>