iio: dac: ad5766: Drop duplicate setting of iio_dev.dev.parent and of_node

This is set to the same value in devm_iio_device_alloc() so no need to do
it again.

Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Cc: Cristian Pop <cristian.pop@analog.com>
Reviewed-by: Alexandru Ardelean <ardeleanalex@gmail.com>
Link: https://lore.kernel.org/r/20210426170251.351957-3-jic23@kernel.org