Disabling a PWM (i.e. calling pwm_apply_state with .enabled = false)
gives no guarantees what the PWM output does. It might freeze where it
currently is, or go in a High-Z state or drive the active or inactive
state, it might even continue to toggle.

To ensure that the LED gets really disabled, don't disable the PWM even
when .duty_cycle is zero.

This fixes disabling a leds-pwm LED on i.MX28. The PWM on this SoC is
one of those that freezes its output on disable, so if you disable an
LED that is full on, it stays on. If you disable a LED with half
brightness it goes off in 50% of the cases and full on in the other 50%.

Fixes: 41c42ff5 ("leds: simple driver for pwm driven LEDs")
Reported-by: Rogan Dawes <rogan@dawes.za.net>
Reported-by: Fabio Estevam <festevam@denx.de>
Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Reviewed-by: Fabio Estevam <festevam@denx.de>
Link: https://lore.kernel.org/r/20230922192834.1695727-1-u.kleine-koenig@pengutronix.deSigned-off-by: Lee Jones <lee@kernel.org>