merge LPTICKER fixes from official ARMmbed#8771

Author: mfatiga

targets/TARGET_STM/lp_ticker.c

@@ -151,7 +151,10 @@ void lp_ticker_init(void)
     HAL_LPTIM_Counter_Start(&LptimHandle, 0xFFFF);
 
     /* Need to write a compare value in order to get LPTIM_FLAG_CMPOK in set_interrupt */
+    __HAL_LPTIM_CLEAR_FLAG(&LptimHandle, LPTIM_FLAG_CMPOK);
     __HAL_LPTIM_COMPARE_SET(&LptimHandle, 0);
+    while (__HAL_LPTIM_GET_FLAG(&LptimHandle, LPTIM_FLAG_CMPOK) == RESET) {
+    }
 }
 
 static void LPTIM1_IRQHandler(void)
@@ -200,10 +203,8 @@ void lp_ticker_set_interrupt(timestamp_t timestamp)
 
     /* CMPOK is set by hardware to inform application that the APB bus write operation to the LPTIM_CMP register has been successfully completed */
     /* Any successive write before the CMPOK flag be set, will lead to unpredictable results */
-    if (__HAL_LPTIM_GET_FLAG(&LptimHandle, LPTIM_FLAG_CMPOK) == RESET) {
-        return;
-    }
-
+    /* LPTICKER_DELAY_TICKS value prevents OS to call this set interrupt function before CMPOK */
+    MBED_ASSERT(__HAL_LPTIM_GET_FLAG(&LptimHandle, LPTIM_FLAG_CMPOK) == SET);
     __HAL_LPTIM_CLEAR_FLAG(&LptimHandle, LPTIM_FLAG_CMPOK);
     __HAL_LPTIM_COMPARE_SET(&LptimHandle, timestamp);