iio: dac: mcp4821: Switch to including mod_devicetable.h for struct of_device_id definition.

of.h was only included for this definition, so include the correct header
instead.

Reviewed-by: Nuno Sa <nuno.sa@analog.com>
Reviewed-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Link: https://lore.kernel.org/r/20240218173323.1023703-9-jic23@kernel.org


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