ipack: split ipack_device_register() in several functions

One function is ipack_device_init(). If it fails, the caller should execute
ipack_put_device().

The second function is ipack_device_add that only adds the device. If
it fails, the caller should execute ipack_put_device().

Then the device is removed with refcount = 0, as device_register() kernel
documentation says.

ipack_device_del() is added to remove the device.

Signed-off-by: Samuel Iglesias Gonsalvez <siglesias@igalia.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>