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Set the read_label() callback to return a friendly name provided in DT
(firmware), in order to make in_{therm,voltage}X_label attributes show
up in sysfs for userspace to consume a channel name.  This is
particularly useful for custom thermistors being attached to otherwise
generically named GPIOs, where the name is known by the board DT.

If the channel name isn't set in DT, use the datasheet_name hardcoded in
the driver instead.

Note that this doesn't fall back to fwnode_get_name() as that provides
suboptimally readable names, with an @xx address suffix from board DT.
Signed-off-by: Marijn Suijten <marijn.suijten@somainline.org>
Reviewed-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>
Link: https://lore.kernel.org/r/20230502-iio-adc-propagate-fw-node-label-v3-5-6be5db6e6b5a@somainline.org

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