hwmon: (pwm-fan) Set fan speed to 0 on suspend

hwmon: (pwm-fan) Set fan speed to 0 on suspend

Technically this is not required because disabling the PWM should be
enough. However, when support for atomic operations was implemented in
the PWM subsystem, only actual changes to the PWM channel are applied
during pwm_config(), which means that during after resume from suspend
the old settings won't be applied.

One possible solution is for the PWM driver to implement its own PM
operations such that settings from before suspend get applied on resume.
This has the disadvantage of completely ignoring any particular ordering
requirements that PWM user drivers might have, so it is best to leave it
up to the user drivers to apply the settings that they want at the
appropriate time.

Another way to solve this would be to read back the current state of the
PWM at the time of resume. That way, in case the configuration was lost
during suspend, applying the old settings in PWM user drivers would
actually get them applied because they differ from the current settings.
However, not all PWM drivers support reading the hardware state, and not
all hardware may support it.

The best workaround at this point seems to be to let PWM user drivers
tell the PWM subsystem that the PWM is turned off by, in addition to
disabling it, also setting the duty cycle to 0. This causes the resume
operation to apply a configuration that is different from the current
configuration, resulting in the proper state from before suspend getting
restored.

Signed-off-by: Thierry Reding <treding@nvidia.com>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>