[Bluetooth] Switch to using input_dev->dev.parent

In preparation for struct class_device -> struct device input core
conversion, switch to using input_dev->dev.parent when specifying
device position in sysfs tree.

Also, do not access input_dev->private directly, use helpers and
do not use kfree() on input device, use input_free_device() instead.

Signed-off-by: Dmitry Torokhov <dtor@mail.ru>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>