USB: Onetouch - 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.

Signed-off-by: Dmitry Torokhov <dtor@mail.ru>
Signed-off-by: Matthew Dharm <mdharm-usb@one-eyed-alien.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>