sysctl: define upper limit for sched_freq_reporting_policy

Setting sched_freq_reporting_policy tunable to an unsupported
values results in a warning from the scheduler. The previous
policy setting is also lost.

As sched_freq_reporting_policy can not be set to an incorrect
value now, remove the WARN_ON_ONCE from the scheduler.

Change-Id: I58d7e5dfefb7d11d2309bc05a1dd66acdc11b766
Signed-off-by: Pavankumar Kondeti <pkondeti@codeaurora.org>