sched: Use one variable for tracking energy efficient cpu

To find energy efficient cpu, target_cpu and energy_cpu variables are
used to track efficient cpu, which is to make sure to return prev_cpu
instead of -1 in case efficient cpu not found.

Use target_cpu alone for making sure to not return -1.

Change-Id: I25bf7bb1996e5f2ed53d850d2030a9aecb5d0cfc
Signed-off-by: Lingutla Chandrasekhar <clingutla@codeaurora.org>