iio: adc: qcom-spmi-adc5: Fall back to datasheet_name instead of fwnode name

Since the migration to fwnode_get_name in commit 4f47a236 ("iio:
adc: qcom-spmi-adc5: convert to device properties") the resulting
adc5_channel_prop::channel_name (renamed from datasheet_name in the
previous patch) - which is propagated into iio_chan_spec::extend_name -
was containing the DT node name including @xx suffix if a "label"
property is not present, while adc5_channels::datasheet_name was thus
far set by the macros but always remained unread.  Put it to use instead
of using a confusing name containing @xx in sysfs filenames (again, when
"label" is not set).

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-v3-3-6be5db6e6b5a@somainline.org


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