leds: leds-pwm: Simplify cleanup code

The code looks more nicer if we use:

while (i--)

instead:
if (i > 0)
	for (i = i - 1; i >= 0; i--)

Signed-off-by: Peter Ujfalusi <peter.ujfalusi@ti.com>
Signed-off-by: Bryan Wu <cooloney@gmail.com>