app/testpmd: fix commands for some offloads

app/testpmd: fix commands for some offloads

Without this patch, testpmd command to config Rx offload keep_crc
would fail and report "Bad argument".
This patch also fix the command to config the Tx offload mbuf_fast_free.

Fixes: 70815c9ecadd ("ethdev: add new offload flag to keep CRC")
Fixes: c73a9071877a ("app/testpmd: add commands to test new offload API")
Cc: stable@dpdk.org
Signed-off-by: Wei Dai <wei.dai@intel.com>
Tested-by: Yuan Peng <yuan.peng@intel.com>
Acked-by: Bernard Iremonger <bernard.iremonger@intel.com>