hwmon: (adt7475) fan stall prevention

By default adt7475 will stop the fans (pwm duty cycle 0%) when the
temperature drops past Tmin - hysteresis. Some systems want to keep the
fans moving even when the temperature drops so add new sysfs attributes
that configure the enhanced acoustics min 1-3 which allows the fans to
run at the minimum configure pwm duty cycle.

Signed-off-by: Chris Packham <chris.packham@alliedtelesis.co.nz>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>

Documentation/hwmon/adt7475

@@ -109,6 +109,11 @@ fan speed) is applied. PWM values range from 0 (off) to 255 (full speed).
 Fan speed may be set to maximum when the temperature sensor associated with
 the PWM control exceeds temp#_max.
 
+At Tmin - hysteresis the PWM output can either be off (0% duty cycle) or at the
+minimum (i.e. auto_point1_pwm). This behaviour can be configured using the
+pwm[1-*]_stall_disable sysfs attribute. A value of 0 means the fans will shut
+off. A value of 1 means the fans will run at auto_point1_pwm.
+
 Notes
 -----