iio:adc:ingenic: drop of_match_ptr protection and include mod_devicetable.h

Whilst it's unlikely this device will ever be instantiated using
ACPI, there is little advantage not assigning the of_device_id table in all
cases and it avoids providing an example that might be coppied into new
drivers.

Also include mod_devicetable.h whilst here as of_device_id is defined
in there so including the header is best practice.

Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Cc: Artur Rojek <contact@artur-rojek.eu>
Reviewed-by: Andy Shevchenko <andy.shevchenko@gmail.com>