First: there are 6 LEDs, not just 4. If the LEDs don't exist, the value is
UNCHANGED on the read request.
The value is more than a simple on/off, though at least the G500s only looks
at the lower two bits, so heartbeat and slow off both translate into "on" and
blink and slow on both translate to "off".
There doesn't seem to be a detection mechanism for what a device really
supports.
Signed-off-by: Peter Hutterer <peter.hutterer@who-t.net>