iio:potentiometer:mcp4131: Drop of_match_ptr and use generic fw interfaces.

This change allows the use of the driver with ACPI via PRP0001
and remove an example of an anti pattern I'm trying to remove from IIO.
Also adjust includes to reflect this change.

Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Reviewed-by: Andy Shevchenko <andy.shevchenko@gmail.com>
Cc: Slawomir Stepien <sst@poczta.fm>
Link: https://lore.kernel.org/r/20200910173242.621168-8-jic23@kernel.org