sched/walt: Fix SCHED_CPUFREQ_CONTINUE for hotplug cases

cluster->cpus might include hotplugged cores and as a result we would
end up sending incorrect flag to cpufreq. Fix the bug by checking
cpu_online_mask.

Change-Id: I39fa9296aac72604c91470a8ffe0a5b9efbaac1f
Signed-off-by: Puja Gupta <pujag@codeaurora.org>