hwmon: (lm90) Read the channel's label from device-tree

Try to read the channel's label from device-tree. Having label in
device-tree node is not mandatory.

Signed-off-by: Slawomir Stepien <slawomir.stepien@nokia.com>
Link: https://lore.kernel.org/r/20220525073657.573327-7-sst@poczta.fm


Signed-off-by: Guenter Roeck <linux@roeck-us.net>