staging: greybus: lights: avoid NULL deref

In the Linux kernel, the following vulnerability has been resolved: staging: greybus: lights: avoid NULL deref gb_lights_light_config() stores channel_count before allocating the channels array. If kcalloc() fails, gb_lights_release() iterates the non-zero count and dereferences light->channels, which is NULL. Allocate channels first and only then publish channels_count so the cleanup path can't walk a NULL pointer.