selftests/resctrl: Change function comments to say < 0 on error

selftests/resctrl: Change function comments to say < 0 on error

A number function comments state the function return non-zero on
failure but in reality they can only return 0 on success and < 0 on
error.

Update the comments to say < 0 on error to match the behavior.

While at it, improve cat_val() comment to state that 0 means the test
was run (either pass or fail).

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>