app/testpmd: fix DCB config

app/testpmd: fix DCB config

After adding RSS hash offload check, default rss_hf will fail on devices
that do not support all bits. This will lead to dcb config failure. The
patch fixes this issue by reading current valid rss_conf from the device.

Fixes: 8863a1fbfc66 ("ethdev: add supported hash function check")
Fixes: 1a572499beb6 ("app/testpmd: setup DCB forwarding based on traffic class")
Cc: stable@dpdk.org
Signed-off-by: Konstantin Ananyev <konstantin.ananyev@intel.com>
Acked-by: Bernard Iremonger <bernard.iremonger@intel.com>