selftests/resctrl: Fix incorrect error return on test complete

An error snuck in between two recent conflicting changes:
Until recently ->setup() used negative values to indicate
normal test termination. This was changed in
commit fa10366c ("selftests/resctrl: Allow ->setup() to return
errors") that transitioned ->setup() to use negative values
to indicate errors and a new END_OF_TESTS to indicate normal
termination.

commit 42e3b093 ("selftests/resctrl: Fix set up schemata with 100%
allocation on first run in MBM test") continued to use
negative return to indicate normal test termination.

Fix mbm_setup() to use the new END_OF_TESTS to indicate
error-free test termination.

Fixes: 42e3b093 ("selftests/resctrl: Fix set up schemata with 100% allocation on first run in MBM test")
Reported-by: Ilpo Järvinen <ilpo.jarvinen@linux.intel.com>
Link: https://lore.kernel.org/lkml/bb65cce8-54d7-68c5-ef19-3364ec95392a@linux.intel.com/


Signed-off-by: Reinette Chatre <reinette.chatre@intel.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>