drivers: macintosh: rack-meter: limit idle ticks to total ticks

Limit idle ticks to total ticks. This prevents the annoying rackmeter
leds fully ON / OFF blinking state that happens on fully idling
G5 Xserve systems.

Signed-off-by: Aaro Koskinen <aaro.koskinen@iki.fi>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>