app/testpmd: fix uninitialized members when setting PFC

Only a part of members in the local structure variable named pfc_conf
are initialized in the function named cmd_priority_flow_ctrl_set_parsed
when typing "set pfc_ctrl..." command, and others are random values.
However, those uninitialized members may cause failure.

This patch adds clearing zero operation before calling the API named
rte_eth_dev_priority_flow_ctrl_set API with pfc_conf as the input
parameter.

Fixes: 9b53e542e9e1 ("app/testpmd: add priority flow control")
Cc: stable@dpdk.org
Signed-off-by: Huisong Li <lihuisong@huawei.com>
Signed-off-by: Wei Hu (Xavier) <xavier.huwei@huawei.com>
Signed-off-by: Xuan Li <lixuan47@hisilicon.com>
Reviewed-by: Ferruh Yigit <ferruh.yigit@intel.com>