iio:potentiometer:ds1803: 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.
Drop them to remove this restriction.

Also switch of.h for mod_devicetable.h include given use of
struct of_device_id which is defined in that header.

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/20200910173242.621168-3-jic23@kernel.org