sched/fair: fix issue with target_cpu in find_best_target()

When we can't get start_cpu or start_sd in find_best_target(), we end up
setting target_cpu (set to -1) as part of the cpus mask which would
later used by find_energy_efficient_cpu() and leads to data abort.

Change-Id: Ifddedba7302294fca2bac945bd9e3464b8f664a1
Signed-off-by: Satya Durga Srinivasu Prabhala <satyap@codeaurora.org>