HID: wacom: leds: actually release the LEDs on disconnect

There is a bug (?) in devm_led_classdev_register() in which its increments
the refcount of the parent. If the parent is an input device, that means
the ref count never reaches 0 when devm_input_device_release() gets called.
This means that the LEDs and all the devres resources attached to the
input device are not released.

Manually force the release of the group so that the leds are released once
we are done using them.

Signed-off-by: Benjamin Tissoires <benjamin.tissoires@redhat.com>
Acked-by: Ping Cheng <pingc@wacom.com>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>