bonding: show Tx policy for 802.3AD mode

bonding: show Tx policy for 802.3AD mode

As balance xmit policy is supported in bonding mode 4(802.3AD). This
patch adds balance xmit policy show in testpmd commands for mode 4. Like:
testpmd> show bonding config 2
        Bonding mode: 4
        Balance Xmit Policy: BALANCE_XMIT_POLICY_LAYER34
        IEEE802.3AD Aggregator Mode: stable
        Slaves (2): [0 1]
        Active Slaves (2): [1 0]
        Primary: [1]

Signed-off-by: Min Hu (Connor) <humin29@huawei.com>
Acked-by: Ferruh Yigit <ferruh.yigit@intel.com>