STM32 RTC init

When LSE is configured for RTC, LSI is not affected

Author: jeromecoutant

targets/TARGET_STM/rtc_api.c

@@ -58,10 +58,9 @@ void rtc_init(void)
     }
 
 #if MBED_CONF_TARGET_LSE_AVAILABLE
-    RCC_OscInitStruct.OscillatorType = RCC_OSCILLATORTYPE_LSI | RCC_OSCILLATORTYPE_LSE;
+    RCC_OscInitStruct.OscillatorType = RCC_OSCILLATORTYPE_LSE;
     RCC_OscInitStruct.PLL.PLLState   = RCC_PLL_NONE; // Mandatory, otherwise the PLL is reconfigured!
     RCC_OscInitStruct.LSEState       = RCC_LSE_ON;
-    RCC_OscInitStruct.LSIState       = RCC_LSI_OFF;
 
     if (HAL_RCC_OscConfig(&RCC_OscInitStruct) != HAL_OK) {
         error("Cannot initialize RTC with LSE\n");
@@ -81,9 +80,8 @@ void rtc_init(void)
     __HAL_RCC_BACKUPRESET_RELEASE();
 
     // Enable LSI clock
-    RCC_OscInitStruct.OscillatorType = RCC_OSCILLATORTYPE_LSI | RCC_OSCILLATORTYPE_LSE;
+    RCC_OscInitStruct.OscillatorType = RCC_OSCILLATORTYPE_LSI;
     RCC_OscInitStruct.PLL.PLLState   = RCC_PLL_NONE; // Mandatory, otherwise the PLL is reconfigured!
-    RCC_OscInitStruct.LSEState       = RCC_LSE_OFF;
     RCC_OscInitStruct.LSIState       = RCC_LSI_ON;
     if (HAL_RCC_OscConfig(&RCC_OscInitStruct) != HAL_OK) {
         error("Cannot initialize RTC with LSI\n");