sched/fair: fix comparision for unisolated candidate

commit 1117bade ("sched: Avoid
placing task on isolated prev_cpu") was intended to reset
the target_cpu when there was no cpu found and prev_cpu was
isolated.

However there is an error where instead of assignment it ends
up comparing with the isolated candidate.

Fix it.

Change-Id: Ia80a89c1074477da9c957eb870fafa7d209386b9
Signed-off-by: Abhijeet Dharmapurikar <adharmap@codeaurora.org>