selftests/resctrl: Remove duplicated preparation for span arg

When no benchmark_cmd is given, benchmark_cmd[1] is set to span in
main(). There's no need to do it again in run_mba_test().

Remove the duplicated preparation for span argument into
benchmark_cmd[1] from run_mba_test(). After this, the has_ben and span
arguments to run_mba_test() can be removed.

Co-developed-by: Fenghua Yu <fenghua.yu@intel.com>
Signed-off-by: Fenghua Yu <fenghua.yu@intel.com>
Signed-off-by: Ilpo Järvinen <ilpo.jarvinen@linux.intel.com>
Reviewed-by: Reinette Chatre <reinette.chatre@intel.com>
Tested-by: Babu Moger <babu.moger@amd.com>
Tested-by: Shaopeng Tan (Fujitsu) <tan.shaopeng@fujitsu.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>