PM / devfreq: fix scaling down logic for simple clock scaling

When "simple_scaling" flag is enabled for on demand governor then clocks
should be scaled up when the load is more than up threshold and should
be scaled down when load is less than the up threshold minus down
differential threshold. But currently governor is only scaling down
when load is less than the down differential threshold which is definitely
not intentional. This change fixes the above bug.

Change-Id: If2a234155c12989dc0df397cd84eef4a759ecdfc
Signed-off-by: Subhash Jadavani <subhashj@codeaurora.org>
[junjiew@codeaurora.org: resolved trivial conflicts]
Signed-off-by: Junjie Wu <junjiew@codeaurora.org>