sched: use group_first_cpu() instead of cpumask_first(sched_group_cpus())

Impact: cleanup

This patch changes cpumask_first(sched_group_cpus()) to group_first_cpu()
for maintainability.

Signed-off-by: Miao Xie <miaox@cn.fujitsu.com>
Acked-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
Signed-off-by: Ingo Molnar <mingo@elte.hu>