platform/x86: thinkpad_acpi: Call led_classdev_notify_brightness_hw_changed on kbd brightness change

Make thinkpad_acpi call led_classdev_notify_brightness_hw_changed on the
kbd_led led_classdev registered by thinkpad_acpi when the kbd backlight
brightness is changed through the hotkey.

This will allow userspace to monitor (poll) for brightness changes on
these LEDs caused by the hotkey.
Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Acked-by: Henrique de Moraes Holschuh <hmh@hmh.eng.br>
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Acked-by: Pavel Machek <pavel@ucw.cz>