Remove unreachable code

IAR generates warning Pe111 'statement is unreachable'. In a couple of
cases, replace return statements with TU_ATTR_FALLTHROUGH; because some
compilers apparently can't figure out that the return statements are
unreachable but do whinge about an imagined fall-through without them!

examples/device/board_test/src/main.c

@@ -67,8 +67,6 @@ int main(void)
       led_state = 1 - led_state; // toggle
     }
   }
-
-  return 0;
 }
 
 #if CFG_TUSB_MCU == OPT_MCU_ESP32S2 || CFG_TUSB_MCU == OPT_MCU_ESP32S3