leds: lp8860: Check return value of devm_mutex_init()

devm_mutex_init() can fail. With CONFIG_DEBUG_MUTEXES=y the mutex will be
marked as unusable and trigger errors on usage.

Add the missed check.

Fixes: 87a59548af95 ("leds: lp8860: Use new mutex guards to cleanup function exits")
Signed-off-by: Thomas Weißschuh <[e-mail]>
Acked-by: Andrew Davis <[e-mail]>
Acked-by: Lee Jones <[e-mail]>
Signed-off-by: Boqun Feng <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]

drivers/leds/leds-lp8860.c

···
 	led->client = client;
 	led->led_dev.brightness_set_blocking = lp8860_brightness_set;
 
-	devm_mutex_init(&client->dev, &led->lock);
+	ret = devm_mutex_init(&client->dev, &led->lock);
+	if (ret)
+		return dev_err_probe(&client->dev, ret, "Failed to initialize lock\n");
 
 	led->regmap = devm_regmap_init_i2c(client, &lp8860_regmap_config);
 	if (IS_ERR(led->regmap)) {