iio:dac:dpot-dac: Swap of.h for mod_devicetable.h

This driver never used anything in the of specific header.
It just wants the struct of_device_id from mod_devicetable.h.

Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Reviewed-by: Andy Shevchenko <andy.shevchenko@gmail.com>
Acked-by: Peter Rosin <peda@axentia.se>