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 <[e-mail]>
Reviewed-by: Andy Shevchenko <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Jonathan Cameron <[e-mail]>

drivers/iio/dac/mcp4821.c

···
  */
 
 #include <linux/module.h>
-#include <linux/of.h>
+#include <linux/mod_devicetable.h>
 #include <linux/spi/spi.h>
 
 #include <linux/iio/iio.h>