iio: dac: ad5791: Use devm_iio_device_register

iio: dac: ad5791: Use devm_iio_device_register

Use devm_iio_device_register to automatically free the iio device.
since this is the last remaining resource that was not automatically
freed, we can drop the ".remove" callback.

Suggested-by: David Lechner <dlechner@baylibre.com>
Reviewed-by: David Lechner <dlechner@baylibre.com>
Signed-off-by: Axel Haslam <ahaslam@baylibre.com>
Link: https://patch.msgid.link/20241031071746.848694-7-ahaslam@baylibre.com
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>