iio: proximity: sx_common: Allow IIO core to take care of firmware node

IIO core correctly will take care of firmware node if it's not set in
the driver. Drop ACPI and OF specifics from the driver and allow IIO
core to handle this.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Reviewed-by: Gwendal Grignou <gwendal@chromium.org>
Link: https://lore.kernel.org/r/20220615114746.2767-2-andriy.shevchenko@linux.intel.com


Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>