backlight: pwm: Clean-up pwm requested using legacy API

If PWM device is requested by means of legacy API pwm_request(), its
resources are not freed on module unbind, which may cause an oops on
access, e.g. by reading /sys/kernel/debug/pwm.
Reported-by: NDmitry Eremin-Solenikov <dmitry_eremin@mentor.com>
Signed-off-by: NVladimir Zapolskiy <vladimir_zapolskiy@mentor.com>
Signed-off-by: NLee Jones <lee.jones@linaro.org>

drivers/video/backlight/pwm_bl.c

@@ -34,6 +34,7 @@ struct pwm_bl_data {
 	struct regulator	*power_supply;
 	struct gpio_desc	*enable_gpio;
 	unsigned int		scale;
+	bool			legacy;
 	int			(*notify)(struct device *,
 					  int brightness);
 	void			(*notify_after)(struct device *,
@@ -274,7 +275,7 @@ static int pwm_backlight_probe(struct platform_device *pdev)
 	pb->pwm = devm_pwm_get(&pdev->dev, NULL);
 	if (IS_ERR(pb->pwm)) {
 		dev_err(&pdev->dev, "unable to request PWM, trying legacy API\n");
-
+		pb->legacy = true;
 		pb->pwm = pwm_request(data->pwm_id, "pwm-backlight");
 		if (IS_ERR(pb->pwm)) {
 			dev_err(&pdev->dev, "unable to request legacy PWM\n");
@@ -339,6 +340,8 @@ static int pwm_backlight_remove(struct platform_device *pdev)
 
 	if (pb->exit)
 		pb->exit(&pdev->dev);
+	if (pb->legacy)
+		pwm_free(pb->pwm);
 
 	return 0;
 }