sched: Try to deal with low capacity, fix update_sd_power_savings_stats()

sgs.group_capacity can now be 0, if for some reason
group->__cpu_power happens to be less than SCHED_LOAD_SCALE/2.

In that case, we need the following fix to make it work for
update_sd_power_savings_stats(). That's because both
sum_nr_running and group_capacity are unsigned longs.

Cc: Gautham R Shenoy <ego@in.ibm.com>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Andreas Herrmann <andreas.herrmann3@amd.com>
Cc: Balbir Singh <balbir@in.ibm.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>