Add labels from IIO channels to our channels. This allows userspace to
display more meaningful names instead of "in0" or "temp5".

Although lm-sensors gracefully handles errors when reading channel
labels, the ABI says the label attribute

> Should only be created if the driver has hints about what this voltage
> channel is being used for, and user-space doesn't.

Therefore, we test to see if the channel has a label before
creating the attribute.
Signed-off-by: Sean Anderson <sean.anderson@linux.dev>
Acked-by: Guenter Roeck <linux@roeck-us.net>
Link: https://patch.msgid.link/20240624174601.1527244-3-sean.anderson@linux.devSigned-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>