selftests/resctrl: Change initialize_llc_perf() return type to void

initialize_llc_perf() unconditionally returns 0.

initialize_llc_perf() performs only memory initialization, none of
which can fail.

Change the return type from int to void to accurately reflect that its
return value doesn't need to be checked. Remove the error checking from
the only callsite.

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>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>