iio:potentiometer:mcp4018: Drop of_match_ptr and CONFIG_OF protections.

These prevent use of this driver with ACPI via PRP0001 and are
an example of an anti pattern I'm trying to remove from IIO.
Also use device_get_match_data() rather than devicetree only version.

Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Reviewed-by: Andy Shevchenko <andy.shevchenko@gmail.com>
Cc: Peter Rosin <peda@axentia.se>
Link: https://lore.kernel.org/r/20200910173242.621168-7-jic23@kernel.org