HID: wacom: Unregister sysfs attributes on remove

HID devices can be hotplugged so we should unregister all sysfs attributes when
removing a driver. Otherwise, manually unloading the wacom-driver will not
remove the sysfs attributes. Only when the device is disconnected, they are
removed, eventually.

Signed-off-by: David Herrmann <dh.herrmann@googlemail.com>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>