net/hns3: fix Tx prepare after stop

net/hns3: fix Tx prepare after stop

In some special scenarios, such as TSO scenarios, the user layer may need
to call the tx_pkt_prepare(), and then call tx_pkt_burst() to send packets.
If the return value of tx_pkt_parepare() isn't equal to the numbers of
packets requested to send, warning message may be printed at the user
layer. Currently, tx_pkt_prepare() is assigned to dummy function when
dev_stop() is called in hns3 PMD. At this moment, if user layer continues
to send packets, the warning message will always be printed. So this patch
modifies the address to NULL.

Fixes: 2790c6464725 ("net/hns3: support device reset")
Cc: stable@dpdk.org
Signed-off-by: Huisong Li <lihuisong@huawei.com>
Signed-off-by: Min Hu (Connor) <humin29@huawei.com>