app/testpmd: fix hex string parser support for flow API

app/testpmd: fix hex string parser support for flow API

There is need for users to set configuration of HEX number for RSS
key. The key byte should be pass down as hex number not as char
string. This patch enable cmdline flow parse HEX number,
in order to not using string which pass ASIC number.

Fixes: f4d623f96119 ("app/testpmd: fix missing RSS fields in flow action")
Cc: stable@dpdk.org
Signed-off-by: Wei Zhao <wei.zhao1@intel.com>
Tested-by: Yuan Peng <yuan.peng@intel.com>
Reviewed-by: Ferruh Yigit <ferruh.yigit@intel.com>