leds: mark led_classdev.default_trigger as const

LED classdev core doesn't modify memory pointed by the default_trigger,
so mark it as const and we'll able to pass const char *s without getting
compiler warnings.
Signed-off-by: Anton Vorontsov <avorontsov@ru.mvista.com>
Signed-off-by: Richard Purdie <rpurdie@rpsys.net>

include/linux/leds.h

@@ -48,7 +48,7 @@ struct led_classdev {
 
 	struct device		*dev;
 	struct list_head	 node;			/* LED Device list */
-	char			*default_trigger;	/* Trigger to use */
+	const char		*default_trigger;	/* Trigger to use */
 
 #ifdef CONFIG_LEDS_TRIGGERS
 	/* Protects the trigger data below */