sched: Use bitmask for sched_busy_hysteresis_enable_cpus tunable

The current code is using a bitmap for sched_busy_hysteresis_enable_cpus
tunable. Since the feature is not enabled for any of the CPUs, the
default value is printed as "\n". This is very inconvenient for
user space applications which tries to write a new value and try to
restore the previous value. Like other scheduler tunables, use
the bitmask to avoid this issue.

Change-Id: I0c5989606352be5382dd688602aefd753fb62317
Signed-off-by: Pavankumar Kondeti <pkondeti@codeaurora.org>