iio:proximity:sx_common: Fix device property parsing on DT systems

iio:proximity:sx_common: Fix device property parsing on DT systems

After commit 7a3605bef878 ("iio: sx9310: Support ACPI property") we
started using the 'indio_dev->dev' to extract device properties for
various register settings in sx9310_get_default_reg(). This broke DT
based systems because dev_fwnode() used in the device_property*() APIs
can't find an 'of_node'. That's because the 'indio_dev->dev.of_node'
pointer isn't set until iio_device_register() is called. Set the pointer
earlier, next to where the ACPI companion is set, so that the device
property APIs work on DT systems.

Cc: Gwendal Grignou <gwendal@chromium.org>
Fixes: 7a3605bef878 ("iio: sx9310: Support ACPI property")
Signed-off-by: Stephen Boyd <swboyd@chromium.org>
Reviewed-by: Gwendal Grignou <gwendal@chromium.org>
Link: https://lore.kernel.org/r/20220331210425.3908278-1-swboyd@chromium.org
Cc: <Stable@vger.kernel.org>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>