eeepc-laptop: document fan_pwm conversions

eeepc_get_fan_pwm and eeepc_set_fan_pwm convert the PWM value read from
the fan to a range lmsensors understands. Unfortunately this is only
clear if you are familiar with how lmsensors handles duty cycles.

Introduce two conversion functions that document the goal of these
conversions.

Signed-off-by: Frans Klaver <fransklaver@gmail.com>

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I can imagine people feeling that it is overkill to introduce two new
functions for this. I on the other hand feel that if you need to explain
what happens, you'd rather introduce a function than comments. The
compiler should deal with optimization.

Should we assume familiarity with lmsensors behavior?