Update uart.c

Add interrupt enter and leave

bsp/lpc176x/drivers/uart.c

@@ -60,7 +60,8 @@ void UART0_IRQHandler(void)
 {
     rt_ubase_t level, iir;
     struct rt_uart_lpc *uart = &uart_device;
-
+	/* enter interrupt */ 
+ 	rt_interrupt_enter();
     /* read IIR and clear it */
     iir = LPC_UART->IIR;
 
@@ -94,7 +95,8 @@ void UART0_IRQHandler(void)
     {
         iir = LPC_UART->LSR;    //oe pe fe oe read for clear interrupt
     }
-
+	/* leave interrupt */ 
+ 	rt_interrupt_leave();
     return;
 }