iio: adc: ad7292: Switch from of specific to fwnode property handling

This reduces the wrong of device tree only IIO drivers that might
be copied by converting over this simple case.
Makes use of the new _scoped() handling to automatically release
the fwnode_handle on early exit from the loop.

Cc: Nuno Sá <nuno.sa@analog.com>
Reviewed-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Link: https://lore.kernel.org/r/20240218172731.1023367-5-jic23@kernel.org


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