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.