plat: unix: correct backlight max_brightness path

We are assuming that device has its backlight device symlinked to /sys/class/backlight/backlight so this should be stated.

Additionally any hardcoding should be disallowed and the symlink should be always used.

frontend/drivers/platform_unix.c

@@ -1349,15 +1349,15 @@ static void frontend_unix_set_screen_brightness(int value)
    char *buffer = NULL;
    char svalue[16] = {0};
    unsigned int max_brightness = 100;
-   #if !defined(HAVE_LAKKA_SWITCH)
-   filestream_read_file("/sys/devices/platform/backlight/backlight/backlight/max_brightness",
+
+   /* Device tree should have 'label = "backlight";' if control is desirable */
+   filestream_read_file("/sys/class/backlight/backlight/max_brightness",
                         &buffer, NULL);
    if (buffer)
    {
       sscanf(buffer, "%u", &max_brightness);
       free(buffer);
    }
-   #endif
 
    /* Calculate the brightness */
    value = (value * max_brightness) / 100;