led-uclass: honour ->label field populated by driver's own .bind

If the driver's own .bind method has populated uc_plat->label, don't
override that. This is necessary for an upcoming driver for ti,lp5562,
where the DT binding unfortunately says to use "chan-name" and not
"label".

Reviewed-by: Christian Gmeiner <christian.gmeiner@gmail.com>
Reviewed-by: Marek Vasut <marex@denx.de>
Signed-off-by: Rasmus Villemoes <rasmus.villemoes@prevas.dk>