sched clock: simplify __update_sched_clock()

- return the current clock instead of letting callers
  fetch it from scd->clock

Signed-off-by: Ingo Molnar <mingo@elte.hu>
Acked-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
Acked-by: Mike Galbraith <efault@gmx.de>