iio:potentiometer:max5432: Drop of_match_ptr and use generic fw accessors

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.
Drop them to remove this restriction.

Also switch headers to reflect this change.

Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Reviewed-by: Andy Shevchenko <andy.shevchenko@gmail.com>
Reviewed-by: Martin Kaiser <martin@kaiser.cx>
Link: https://lore.kernel.org/r/20200910173242.621168-4-jic23@kernel.org