iio:potentiometer:max5481: 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.
Whilst this driver has an ACPI binding, it is not of a form
that is valid under ACPI so will be dropped shortly.

Also switch to device_get_match_data() and switch headers.

Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Reviewed-by: Andy Shevchenko <andy.shevchenko@gmail.com>
Cc: Maury Anderson <maury.anderson@rockwellcollins.com>
Cc: Matthew Weber <matthew.weber@rockwellcollins.com>
Cc: Slawomir Stepien <sst@poczta.fm>
Link: https://lore.kernel.org/r/20200910173242.621168-5-jic23@kernel.org