sched/fair: Simplify idle_idx handling in select_idle_sibling()

sched/fair: Simplify idle_idx handling in select_idle_sibling()

Rename best_idle to best_idle_cpu so the same name is used like in
find_best_target().

Fix if (best_idle > 0) since best_idle_cpu = 0 is a valid target.

Use 'unsigned long' data type for best_idle_capacity.

Since we're looking for the shallowest best_idle_cstate initialize
best_idle_cstate = INT_MAX. For cpus which are not idle (idle_idx = -1)
the condition 'if (idle_idx < best_idle_cstate && ...)' is never
executed.

Change-Id: Ic5b63d58478696b3d1ec6253cf739a69a574cf99
Signed-off-by: Dietmar Eggemann <dietmar.eggemann@arm.com>
(cherry picked from commit 8bff5e9c0968108d465e1f2a4624fc5ec2f00849)
Signed-off-by: Chris Redpath <chris.redpath@arm.com>