iio:adc:mcp3422: remove CONFIG_OF and of_match_ptr protections

They stop the driver being used with ACPI PRP0001 and are something
I want to avoid being cut and paste into new drivers.
Also switch the include from of.h to mod_devicetable.h as we
struct of_device_id is defined in there and we don't use anything
actually in of.h.

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