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