selftests/resctrl: Correct get_llc_perf() param in function comment

get_llc_perf() function comment refers to cpu_no parameter that does
not exist.

Correct get_llc_perf() the comment to document llc_perf_miss instead.

Signed-off-by: Ilpo Järvinen <ilpo.jarvinen@linux.intel.com>
Reviewed-by: Reinette Chatre <reinette.chatre@intel.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>