cpufreq conservative:Use an inline function to evaluate

Use an inline function to evaluate freq_target to avoid duplicate code.

Also, define a macro for the default frequency step.

Signed-off-by: Stratos Karafotis <stratosk@semaphore.gr>
Fix the logic in frequency

decrease checking

When we evaluate the CPU load for frequency decrease we have to compare
the load against down_threshold. There is no need to subtract 10 points
from down_threshold.

Instead, we have to use the default down_threshold or user's selection
unmodified.

Signed-off-by: Stratos Karafotis <stratosk@semaphore.gr>
Fix sampling_down_factor

functionality

sampling_down_factor tunable is unused since commit
8e677ce (4 years ago).

This patch restores the original functionality and documents the
tunable.

Signed-off-by: Stratos Karafotis <stratosk@semaphore.gr>