Current the label is retrieved by the line-name property but this is a
optional property. In case of a missing line-name property the label is
NULL. As the binding documentation told, the gpio-label must be set to
the device-node name in case of missing line-name property.

Fixes: 37e6bee7e5 ("gpiolib: Add support for GPIO "hog" nodes")
Signed-off-by: Marco Felsch <m.felsch@pengutronix.de>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>