iio:dac:ad7303: Drop of_match_ptr protection

This prevents use of this driver with ACPI via PRP0001 and are
an example of an anti pattern I'm trying to remove from IIO.

Also add mod_devicetable.h include given struct of_device_id is
declared there.
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Reviewed-by: Andy Shevchenko <andy.shevchenko@gmail.com>
Cc: Michael Hennerich <michael.hennerich@analog.com>
Cc: Lars-Peter Clausen <lars@metafoo.de>
Link: https://lore.kernel.org/r/20200910173242.621168-13-jic23@kernel.org

drivers/iio/dac/ad7303.c

@@ -7,6 +7,7 @@
 
 #include <linux/err.h>
 #include <linux/module.h>
+#include <linux/mod_devicetable.h>
 #include <linux/kernel.h>
 #include <linux/spi/spi.h>
 #include <linux/slab.h>
@@ -287,7 +288,7 @@ MODULE_DEVICE_TABLE(spi, ad7303_spi_ids);
 static struct spi_driver ad7303_driver = {
 	.driver = {
 		.name = "ad7303",
-		.of_match_table = of_match_ptr(ad7303_spi_of_match),
+		.of_match_table = ad7303_spi_of_match,
 	},
 	.probe = ad7303_probe,
 	.remove = ad7303_remove,