staging:iio:fix using iio_priv or iio_dev after iio_unregister_device

This includes calling iio_free_device as that should only be called
before the device has been registered.

Also one case of iio_free_device being called on a registered device.

This has been pulled out of the reworking attributes patch.

Signed-off-by: Jonathan Cameron <jic23@cam.ac.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>