iio:potentiometer:mcp4531: 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.
Hence drop them from this driver.

Also switch to device_get_match_data() from of_ variant and adjust
headers to reflect the change.

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/20200910173242.621168-9-jic23@kernel.org