iio:adc:mcp320x: Drop CONFIG_OF and of_match_ptr protections

These just prevent the driver being used with ACPI PRP0001.
They also get cut and paste into new drivers and we want to discourage
this from happening by cleaning it out in general.

Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Cc: Oskar Andero <oskar.andero@gmail.com>
Cc: Søren Andersen <san@rosetechnology.dk>
Cc: Lukas Wunner <lukas@wunner.de>
Reviewed-by: Andy Shevchenko <andy.shevchenko@gmail.com>