app/testpmd: fix forward lcores number for DCB

app/testpmd: fix forward lcores number for DCB

For the DCB forwarding test, each core is assigned to each traffic class.
Number of forwarding cores for DCB test must be equal or less than number
of total TC. Otherwise, the following problems may occur:
1/ Redundant polling threads will be created when forwarding cores number
   is greater than total TC number.
2/ Two cores would try to use a same queue on a port when Rx/Tx queue
   number is greater than the used TC number, which is not allowed.

Fixes: 900550de04a7 ("app/testpmd: add dcb support")
Fixes: ce8d561418d4 ("app/testpmd: add port configuration settings")
Cc: stable@dpdk.org
Signed-off-by: Huisong Li <lihuisong@huawei.com>
Signed-off-by: Lijun Ou <oulijun@huawei.com>
Acked-by: Xiaoyun Li <xiaoyun.li@intel.com>