sched/fair: Fix incorrect CPU access in check_for_migration()

Commit 3a94de0b ("sched/fair: Prevent tick path active migration
to the same CPU") removed the -1 check for the return value of
find_energy_efficient_cpu() while adding the check against the
previous CPU. If the new CPU is -1, then it passes the check of
new_cpu != prev_cpu and end up calling capacity_orig_of() with
-1 as an argument. Fix this issue by adding the -1 check again.

Change-Id: I6768ef722f25885f97583e493a6753b5e3fe28ee
Signed-off-by: Pavankumar Kondeti <pkondeti@codeaurora.org>