liblights: Fix variable button brightness code

* Because we're using #ifdef in set_light_buttons(),
  defining VAR_BUTTON_BRIGHTNESS to anything, including 0,
  results in the button brightness code being enabled
* That results in broken touchkey backlight for devices
  that don't override the samsung_lights header, or defined
  this macro to 0 in their device's samsung_lights header
* Comment out the macro to correct this logic

Change-Id: Icf554b0deac975f729f1a70e517341dae74f3657

liblights/include/samsung_lights.h

@@ -30,8 +30,8 @@
 #define BUTTON_BRIGHTNESS_NODE "/sys/class/sec/sec_touchkey/brightness"
 #define LED_BLINK_NODE "/sys/class/sec/led/led_blink"
 
-// Set to 1 to enable variable button brightness
-#define VAR_BUTTON_BRIGHTNESS 0
+// Uncomment to enable variable button brightness
+//#define VAR_BUTTON_BRIGHTNESS 1
 
 /*
  * Brightness adjustment factors