sched: make sched_set_window() return failure when PELT is in use

Window-based load tracking is a pre-requisite for the scheduler to
feed cpu load information to the governor. When PELT is in use, return
failure when governor attempts to set window-size. This will let
governor fall back to other APIs for retrieving cpu load statistics.

Change-Id: I0e11188594c1a54b3b7ff55447d30bfed1a01115
Signed-off-by: Srivatsa Vaddagiri <vatsa@codeaurora.org>
Signed-off-by: Syed Rameez Mustafa <rameezmustafa@codeaurora.org>