sched: Allow __sched_setscheduler call from interrupt context if not pi

BUG() is only needed for pi case which needed
call to a rtmutex function and grab a spin_lock.
While in pi case is a simplified logic to normalize
tasks, it is safe to be called from interrupt
context.

CRs-Fixed: 2101157
Change-Id: Icdcb9356692fc30d7b5af40f816381df5b0f0d58
Signed-off-by: Maria Yu <aiquny@codeaurora.org>