iio: adc: ad7192: Convert from of specific to fwnode property handling

Enables use of with other firmwware types.
Removes a case of device tree specific handlers that might get copied
into new drivers.

Cc: Alisa-Dariana Roman <alisa.roman@analog.com>
Reviewed-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Link: https://lore.kernel.org/r/20240218172731.1023367-6-jic23@kernel.org


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