iio: Add iio_read_channel_label to inkern API

iio: Add iio_read_channel_label to inkern API

It can be convenient for other in-kernel drivers to reuse IIO channel
labels. Export the iio_read_channel_label function to allow this. The
signature is different depending on where we are calling it from, so
the meat is moved to do_iio_read_channel_label.

Signed-off-by: Sean Anderson <sean.anderson@linux.dev>
Acked-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Link: https://patch.msgid.link/20240624174601.1527244-2-sean.anderson@linux.dev
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>