greybus: lights: remove unnecessary check

lights can never be NULL at that point since lights_count must be different than
zero, and we need only to validate the light_id.

Reported-by: Johan Hovold <johan@hovoldconsulting.com>
Signed-off-by: Rui Miguel Silva <rui.silva@linaro.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>