buqfreq: Use cpu_load_slope correctly

The load slope is the delta from the current load to the last cpu load sample.
It's only positive if the current load exceeds the last load, so why is it scaling
down a step if the load is actually higher? It makes no sense.

Change it to correctly scale down a step if the load slope exceeds the load
slope size.

Signed-off-by: Mohamed Karami <karamimed@gmail.com>