STM32F3 - Fix issue with MBED_16+IAR

Author: bcostm

hal/targets/hal/TARGET_STM/TARGET_STM32F3/rtc_api.c

@@ -99,6 +99,7 @@ void rtc_init(void)
 #endif
 
 #if DEVICE_RTC_LSI
+    if (rtc_inited) return;
     rtc_inited = 1;
 #endif