cpufreq: schedutil: update warn_on with bug_on

WARN_ON is used in calculating schedutil average capacity
this is called under rq->lock held, when WARN_ON is triggered
it tries to wake_up the process on the same CPU for debug
prints inturn waits for the rq->lock triggering dead lock

Update WARN_ON with BUG_ON avoiding the debug prints.

Change-Id: I8db35a2165e68765b4ab2f132a571ad00311ba25
Signed-off-by: Santosh Mardi <gsantosh@codeaurora.org>