HID: wacom: convert LEDs to devres

Use our own wacom_devm_sysfs_create_group() as there is currently no
generic one. It has been requested at least twice [1][2] but has been
always rejected.
However, in the Wacom case, for the wirelessly connected devices, we need
to be able to release the created sysfs files without removing the parent
kobject.

[1] https://patchwork.kernel.org/patch/7526551/
[2] https://lkml.org/lkml/2013/3/14/728



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