selftests/resctrl: Check for return value after write_schemata()

MBA test case writes schemata but it does not check if the write is
successful or not.

Add the error check and return error properly.

Fixes: 01fee6b4 ("selftests/resctrl: Add MBA test")
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>