iio: Remove debugfs entries in iio_device_unregister()

Remove the the debugfs entries in iio_device_unregister(). Otherwise the debugfs
entries might still be accessible even though the device used in the debugfs
callback has already been freed.

Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>