sched: fix uninitialized variable usage

Select idle sibling does isolation check for uninitialized variable
instead of prev cpu.

Fix it by replacing the variable with prev_cpu.

Change-Id: Ib4557f583ea7fb7129c930e536138a6924a8cd94
Signed-off-by: Lingutla Chandrasekhar <clingutla@codeaurora.org>
Signed-off-by: Abhijeet Dharmapurikar <adharmap@codeaurora.org>