iio: frequency: ad9523: convert rest of driver to device managed functions

The driver pretty much uses device managed functions. The only left-over is
the iio_device_register() function, which also requires an action-or-reset
hook to disable the regulator on the remove and error path.

Signed-off-by: Alexandru Ardelean <alexandru.ardelean@analog.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>