app/testpmd: fix SW checksum calculation enabling

app/testpmd: fix SW checksum calculation enabling

In current design, we can't enable SW checksum calculation
for the devices which don't have checksum offloading abilities
via the command "csum set ip|tcp|udp|sctp|outer-ip sw <port_id>".
But SW checksum calculation shouldn't depend on HW offloading
abilities. This patch is to fix this issue.

Fixes: 3926dd2b6668 ("app/testpmd: enforce offload capabilities check")
Signed-off-by: Jiayu Hu <jiayu.hu@intel.com>
Reviewed-by: Shahaf Shuler <shahafs@mellanox.com>