HID: appletb-kbd: fix "appletb_backlight" backlight device reference counting

JIRA: https://issues.redhat.com/browse/RHEL-101770
Upstream Status: 6.16-rc
Tested: with the hid-tools test suite and some hardware

commit 4540e41
Author: Qasim Ijaz <qasdev00@gmail.com>
Date:   Sun Jun 15 23:59:41 2025 +0100

    HID: appletb-kbd: fix "appletb_backlight" backlight device reference counting

    During appletb_kbd_probe, probe attempts to get the backlight device
    by name. When this happens backlight_device_get_by_name looks for a
    device in the backlight class which has name "appletb_backlight" and
    upon finding a match it increments the reference count for the device
    and returns it to the caller. However this reference is never released
    leading to a reference leak.

    Fix this by decrementing the backlight device reference count on removal
    via put_device and on probe failure.

    Fixes: 93a0fc4 ("HID: hid-appletb-kbd: add support for automatic brightness control while using the touchbar")
    Cc: stable@vger.kernel.org
    Signed-off-by: Qasim Ijaz <qasdev00@gmail.com>
    Reviewed-by: Aditya Garg <gargaditya08@live.com>
    Signed-off-by: Jiri Kosina <jkosina@suse.com>

Signed-off-by: Benjamin Tissoires <benjamin.tissoires@redhat.com>

Author: bentiss

drivers/hid/hid-appletb-kbd.c

@@ -437,6 +437,8 @@ static int appletb_kbd_probe(struct hid_device *hdev, const struct hid_device_id
 unregister_handler:
 	input_unregister_handler(&kbd->inp_handler);
 close_hw:
+	if (kbd->backlight_dev)
+		put_device(&kbd->backlight_dev->dev);
 	hid_hw_close(hdev);
 stop_hw:
 	hid_hw_stop(hdev);
@@ -452,6 +454,9 @@ static void appletb_kbd_remove(struct hid_device *hdev)
 	input_unregister_handler(&kbd->inp_handler);
 	del_timer_sync(&kbd->inactivity_timer);
 
+	if (kbd->backlight_dev)
+		put_device(&kbd->backlight_dev->dev);
+
 	hid_hw_close(hdev);
 	hid_hw_stop(hdev);
 }