FROMLIST: 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.

Signed-off-by: Valentin Schneider <valentin.schneider@arm.com>
[from https://lore.kernel.org/lkml/1530699470-29808-7-git-send-email-morten.rasmussen@arm.com/

]
Signed-off-by: Valentin Schneider <valentin.schneider@arm.com>
Signed-off-by: Chris Redpath <chris.redpath@arm.com>
Change-Id: I8d3cc862292290c952505ff78e41cfe6b04bf168