greybus: lights: fix check for configured lights

The validation for a complete configured light is wrong and it is
reworked to make sure that only when the light is ready, will handle
request events.

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