sched: Fix a division by zero bug in scale_exec_time()

When cycle_counter is used to estimate the frequency, calling
update_task_ravg() twice on the same task without refreshing
the wallclock results in a division by zero bug. Add a safety
check in update_task_ravg() to prevent this.

The above bug is hit from __schedule() when next == prev. There
is no need to call update_task_ravg() twice for PUT_PREV_TASK
and PICK_NEXT_TASK events for the same task. Calling
update_task_ravg() with TASK_UPDATE event is sufficient.

Change-Id: Ib3af9004f2462618c535b8195377bedb584d0261
Signed-off-by: Pavankumar Kondeti <pkondeti@codeaurora.org>