Updated wrongly defined UART3_BASE into USART3_BASE

Author: helmut64

targets/TARGET_STM/TARGET_STM32L4/serial_device.c

@@ -491,7 +491,7 @@ static IRQn_Type serial_get_irq_n(serial_t *obj)
             irq_n = USART2_IRQn;
             break;
 
-#if defined(UART3_BASE)
+#if defined(USART3_BASE)
         case 2:
             irq_n = USART3_IRQn;
             break;