iio: core: Point users of extend_name field to read_label callback

iio: core: Point users of extend_name field to read_label callback

As mentioned and discussed in [1] extend_name should not be used for
full channel labels (and most drivers seem to only use it to express a
short type of a channel) as this affects sysfs filenames, while the
label name is supposed to be extracted from the *_label sysfs file
instead.  This appears to have been unclear to some drivers as
extend_name is also used when read_label is unset, achieving an initial
goal of providing sensible names in *_label sysfs files without noticing
that sysfs filenames are (negatively and likely unintentionally)
affected as well.

Point readers of iio_chan_spec::extend_name to iio_info::read_label by
mentioning deprecation and side-effects of this field.

[1]: https://lore.kernel.org/linux-arm-msm/20221221223432.si2aasbleiicayfl@SoMainline.org/

Suggested-by: Jonathan Cameron <jic23@kernel.org>
Signed-off-by: Marijn Suijten <marijn.suijten@somainline.org>
Link: https://lore.kernel.org/r/20230502-iio-adc-propagate-fw-node-label-v3-1-6be5db6e6b5a@somainline.org
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>