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>
Signed-off-by: Darren Hart <dvhart@linux.intel.com>