iio:adc:max11100: Drop of_match_ptr protection / add mod_devicetable.h include

Allows use of driver with ACPI PRP0001 base binding.
Mostly this is about trying to avoid cut and paste of this into new
drivers (it is a frequent review comment) rather than any
thought that this driver might get used on an ACPI platform.

The mod_devicetable.h include is to encourage best practice of including
any header directly used within the code (here for of_match_id)

Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Acked-by: Jacopo Mondi <jacopo@jmondi.org>
Reviewed-by: Andy Shevchenko <andy.shevchenko@gmail.com>