Bug fix for MBED_16 on STM32F7 family

Author: adustm

hal/targets/hal/TARGET_STM/TARGET_STM32F7/rtc_api.c

@@ -182,6 +182,7 @@ time_t rtc_read(void)
     timeinfo.tm_hour = timeStruct.Hours;
     timeinfo.tm_min  = timeStruct.Minutes;
     timeinfo.tm_sec  = timeStruct.Seconds;
+    timeinfo.tm_isdst  = -1;
 
     // Convert to timestamp
     time_t t = mktime(&timeinfo);