sched: fix single cpu running issue

When kernel run on single core, the kernel will panic if there
are clusters more than one, because there is a chance that
the value of return from cluster_first_cpu() greater than
num_possible_cpus().

Change-Id: I39f56b25c759af7e241eb083a3ddd2c0383e9d8b
Signed-off-by: Tengfei Fan <tengfeif@codeaurora.org>