iio:adc:max1118: Drop CONFIG_OF / of_match_ptr protections

These just stop the driver being used with ACPI PRP0001 and
we are trying to clear them out of IIO to avoid them getting copied
into new drivers.
Also add the mod_devicetable.h include as we are using
struct of_device_id which is defined in there.

Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Cc: Akinobu Mita <akinobu.mita@gmail.com>
Reviewed-by: Andy Shevchenko <andy.shevchenko@gmail.com>