sched_rt: don't start timer when rt bandwidth disabled

Impact: fix incorrect condition check

No need to start rt bandwidth timer when rt bandwidth is disabled.
If this timer starts, it may stop at sched_rt_period_timer() on the first time.

Signed-off-by: Hiroshi Shimamoto <h-shimamoto@ct.jp.nec.com>
Acked-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
Signed-off-by: Ingo Molnar <mingo@elte.hu>