app/testpmd: retain all original dev conf when config DCB

app/testpmd: retain all original dev conf when config DCB

When configuring DCB, testpmd retains the rx_mode/tx_mode configuration in
rte_port->dev_conf. But some configurations, such as the link_speed, were
not saved if they were set before configuring DCB.

Fixes: 1a572499beb6 ("app/testpmd: setup DCB forwarding based on traffic class")
Cc: stable@dpdk.org
Signed-off-by: Huisong Li <lihuisong@huawei.com>
Signed-off-by: Min Hu (Connor) <humin29@huawei.com>
Acked-by: Xiaoyun Li <xiaoyun.li@intel.com>