[BSP] Fix the interrupt issue in USART driver of STM32F4.

Author: BernardXiong

bsp/stm32f40x/drivers/usart.c

@@ -89,8 +89,6 @@ static rt_err_t stm32_configure(struct rt_serial_device *serial, struct serial_c
 
     /* Enable USART */
     USART_Cmd(uart->uart_device, ENABLE);
-    /* enable interrupt */
-    USART_ITConfig(uart->uart_device, USART_IT_RXNE, ENABLE);
 
     return RT_EOK;
 }
@@ -107,10 +105,14 @@ static rt_err_t stm32_control(struct rt_serial_device *serial, int cmd, void *ar
     case RT_DEVICE_CTRL_CLR_INT:
         /* disable rx irq */
         UART_DISABLE_IRQ(uart->irq);
+        /* disable interrupt */
+        USART_ITConfig(uart->uart_device, USART_IT_RXNE, DISABLE);
         break;
     case RT_DEVICE_CTRL_SET_INT:
         /* enable rx irq */
         UART_ENABLE_IRQ(uart->irq);
+        /* enable interrupt */
+        USART_ITConfig(uart->uart_device, USART_IT_RXNE, ENABLE);
         break;
     }