iio:adc:ti-adc108s102: Drop CONFIG_OF and of_match_ptr protections

I'm trying to clean this (now) anti-pattern out of IIO to avoid
cut and paste into new drivers.

Also add an include of mod_devicetable.h as the driver directly uses
struct of_device_id which is defined in there.

Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Reviewed-by: Andy Shevchenko <andy.shevchenko@gmail.com>
Cc: Jan Kiszka <jan.kiszka@siemens.com>