iio: dac: ad5449: drop support for platform data

There are no longer any users of the platform data struct. Remove
support for it from the driver.

Signed-off-by: Bartosz Golaszewski <bartosz.golaszewski@linaro.org>
Link: https://patch.msgid.link/20240814092629.9862-1-brgl@bgdev.pl


Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>