leds: leds-bcm63138: get rid of LED_OFF

The whole "enum led_brightness" is marked as obsolete. Replace it with a
(non-)zero check.

Reported-by: Pavel Machek <pavel@ucw.cz>
Signed-off-by: Rafał Miłecki <rafal@milecki.pl>
Signed-off-by: Pavel Machek <pavel@ucw.cz>