stm32/rtc: Fix rtc_info flags when LSE fails and falls back to LSI.

Previously, if LSE is selected but fails and the RTC falls back to LSI,
then the rtc_info flags would incorrectly state that LSE is used.  This
patch fixes that by setting the bit in rtc_info only after the clock is
ready.

ports/stm32/rtc.c

@@ -162,7 +162,7 @@ void rtc_init_finalise() {
         return;
     }
 
-    rtc_info = 0x20000000 | (rtc_use_lse << 28);
+    rtc_info = 0x20000000;
     if (PYB_RTC_Init(&RTCHandle) != HAL_OK) {
         if (rtc_use_lse) {
             // fall back to LSI...
@@ -182,6 +182,9 @@ void rtc_init_finalise() {
         }
     }
 
+    // record if LSE or LSI is used
+    rtc_info |= (rtc_use_lse << 28);
+
     // record how long it took for the RTC to start up
     rtc_info |= (HAL_GetTick() - rtc_startup_tick) & 0xffff;