hwmon: (nct6775) Fix platform driver suspend regression

Commit c3963bc0a0 ("hwmon: (nct6775) Split core and platform
driver") introduced a slight change in nct6775_suspend() in order to
avoid an otherwise-needless symbol export for nct6775_update_device(),
replacing a call to that function with a simple dev_get_drvdata()
instead.

As it turns out, there is no guarantee that nct6775_update_device()
is ever called prior to suspend. If this happens, the resume function
ends up writing bad data into the various chip registers, which results
in a crash shortly after resume.

To fix the problem, just add the symbol export and return to using
nct6775_update_device() as was employed previously.

Reported-by: Zoltán Kővágó <dirty.ice.hu@gmail.com>
Tested-by: Zoltán Kővágó <dirty.ice.hu@gmail.com>
Fixes: c3963bc0a0 ("hwmon: (nct6775) Split core and platform driver")
Cc: stable@kernel.org
Signed-off-by: Zev Weiss <zev@bewilderbeest.net>
Link: https://lore.kernel.org/r/20220810052646.13825-1-zev@bewilderbeest.net
[groeck: Updated description]
Signed-off-by: Guenter Roeck <linux@roeck-us.net>

drivers/hwmon/nct6775-core.c

@@ -1480,7 +1480,7 @@ static int nct6775_update_pwm_limits(struct device *dev)
 	return 0;
 }
 
-static struct nct6775_data *nct6775_update_device(struct device *dev)
+struct nct6775_data *nct6775_update_device(struct device *dev)
 {
 	struct nct6775_data *data = dev_get_drvdata(dev);
 	int i, j, err = 0;
@@ -1615,6 +1615,7 @@ out:
 	mutex_unlock(&data->update_lock);
 	return err ? ERR_PTR(err) : data;
 }
+EXPORT_SYMBOL_GPL(nct6775_update_device);
 
 /*
  * Sysfs callback functions

drivers/hwmon/nct6775-platform.c

@@ -359,7 +359,7 @@ static int __maybe_unused nct6775_suspend(struct device *dev)
 {
 	int err;
 	u16 tmp;
-	struct nct6775_data *data = dev_get_drvdata(dev);
+	struct nct6775_data *data = nct6775_update_device(dev);
 
 	if (IS_ERR(data))
 		return PTR_ERR(data);

drivers/hwmon/nct6775.h

@@ -196,6 +196,8 @@ static inline int nct6775_write_value(struct nct6775_data *data, u16 reg, u16 va
 	return regmap_write(data->regmap, reg, value);
 }
 
+struct nct6775_data *nct6775_update_device(struct device *dev);
+
 bool nct6775_reg_is_word_sized(struct nct6775_data *data, u16 reg);
 int nct6775_probe(struct device *dev, struct nct6775_data *data,
 		  const struct regmap_config *regmapcfg);