leds: pca9532: Don't stop blinking for non-zero brightness

pca9532 unexpectedly stopped blinking when changing brightness to a
non-zero value. To reproduce:

 echo timer > /sys/class/leds/led-1/trigger   # blinks
 echo 255 > /sys/class/leds/led-1/brightness  # blinking stops, light on
 cat /sys/class/leds/led-1/trigger            # still claims [timer]

According to Documentation/leds/leds-class.rst, only brightness = 0
shall be a stop condition:

> You can change the brightness value of a LED independently of the
> timer trigger. However, if you set the brightness value to LED_OFF it
> will also disable the timer trigger.

Therefore add a guard to continue blinking when brightness != LED_OFF,
similar to how pca955x does it since 575f10dc64a2 ("leds: pca955x: Add
HW blink support").

Signed-off-by: Tobias Deiminger <tobias.deiminger@linutronix.de>
Link: https://patch.msgid.link/20260331202848.658676-1-tobias.deiminger@linutronix.de
Signed-off-by: Lee Jones <lee@kernel.org>