iio:adc:ti-adc128s052: drop of_match_ptr protection

There is no real advantage in having these protections and
for parts that do not have an explicit ACPI ID, it prevents the
use of PRP0001. I'm trying to clear this out of IIO in general
to avoid copying in new drivers.

Include mod_devicetable.h as we are using of_device_id in here so
including that header is best practice.

Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Reviewed-by: Andy Shevchenko <andy.shevchenko@gmail.com>
Cc: Angelo Compagnucci <angelo.compagnucci@gmail.com>