sched/fair: Change prefer_sibling type to bool

This variable is entirely local to update_sd_lb_stats, so we can
safely change its type and slightly clean up its initialisation.

Cc: vincent.guittot@linaro.org
Cc: mingo@redhat.com
Cc: dietmar.eggemann@arm.com
Cc: gaku.inami.xh@renesas.com
Signed-off-by: Valentin Schneider <valentin.schneider@arm.com>
Signed-off-by: Morten Rasmussen <morten.rasmussen@arm.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: http://lkml.kernel.org/r/1530699470-29808-7-git-send-email-morten.rasmussen@arm.com