[STM32F3] Use USART3_BASE instead of UART3_BASE

This change allows the use of UART3 instance.
This change fixes the following issue :
https://developer.mbed.org/questions/4937/Serial-problem-no-Tx-to-PB_10
It's been validated on NUCLEO_F302RB, for PB_9 and PB_10 pins

Author: adustm

hal/targets/hal/TARGET_STM/TARGET_STM32F3/serial_api.c

@@ -94,7 +94,7 @@ void serial_init(serial_t *obj, PinName tx, PinName rx)
         __HAL_RCC_USART2_CONFIG(RCC_USART2CLKSOURCE_SYSCLK);
         obj->index = 1;
     }
-#if defined(UART3_BASE)
+#if defined(USART3_BASE)
     if (obj->uart == UART_3) {
         __USART3_CLK_ENABLE();
         __HAL_RCC_USART3_CONFIG(RCC_USART3CLKSOURCE_SYSCLK);
@@ -157,7 +157,7 @@ void serial_free(serial_t *obj)
         __USART2_RELEASE_RESET();
         __USART2_CLK_DISABLE();
     }
-#if defined(UART3_BASE)
+#if defined(USART3_BASE)
     if (obj->uart == UART_3) {
         __USART3_FORCE_RESET();
         __USART3_RELEASE_RESET();
@@ -252,7 +252,7 @@ static void uart2_irq(void)
     uart_irq(UART_2, 1);
 }
 
-#if defined(UART3_BASE)
+#if defined(USART3_BASE)
 static void uart3_irq(void)
 {
     uart_irq(UART_3, 2);
@@ -296,7 +296,7 @@ void serial_irq_set(serial_t *obj, SerialIrq irq, uint32_t enable)
         vector = (uint32_t)&uart2_irq;
     }
 
-#if defined(UART3_BASE)
+#if defined(USART3_BASE)
     if (obj->uart == UART_3) {
         irq_n = USART3_IRQn;
         vector = (uint32_t)&uart3_irq;