sched/core: fix possible out-of-bounds access in updown_migrate_values()

In sched_update_updown_migrate_values(), while checking for elements
in cluster_cpus[], there is a possibility that we end up accessing
an element (i = MAX_CLUSTERS) which is out-of-bounds. Fix it by
updating the check.

Change-Id: Ia0587df9cf3add12475918171ea0578d99fa8782
Signed-off-by: Satya Durga Srinivasu Prabhala <satyap@codeaurora.org>