We can't pass a NULL to input_unregister_device().

Fixes: 2a6cdbdd ('HID: wacom: Introduce new 'touch_input' device')
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Reviewed-by: Jason Gerecke <jason.gerecke@wacom.com>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>