HID: wacom: use devres to allocate driver data

We started switching the driver to devres, so we should use it as much
as possible.

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