leds: lm3532: Avoid potentially unpaired regulator calls

We may currently get unpaired regulator calls when configuring the LED
brightness via sysfs in case of regulator calls producing errors. Let's
fix this by maintaining local state for enabled.

Signed-off-by: Tony Lindgren <tony@atomide.com>
Acked-by: Pavel Machek <pavel@ucw.cz>
Signed-off-by: Jacek Anaszewski <jacek.anaszewski@gmail.com>