drivers: led: bcm6858: do not use null label to find the top

This driver considers that a node with an empty label is the top.
But the led class has changed, if a label is not provided for a led,
the label is filed with the node name. So we update this driver
to use a wrapper to manage the top led node.

Signed-off-by: Philippe Reynes <philippe.reynes@softathome.com>