STM32 LPTICKER with LPTIM : Fix tickless and lp wrapper

This fixes issue with mbed_hal/lp_ticker/lp_ticker_early_match_race_test

Author: jeromecoutant

targets/TARGET_STM/lp_ticker.c

@@ -145,9 +145,6 @@ void lp_ticker_init(void)
 
     __HAL_LPTIM_ENABLE_IT(&LptimHandle, LPTIM_IT_CMPM);
     HAL_LPTIM_Counter_Start(&LptimHandle, 0xFFFF);
-
-    /* Need to write a compare value in order to get LPTIM_FLAG_CMPOK in set_interrupt */
-    __HAL_LPTIM_COMPARE_SET(&LptimHandle, 0);
 }
 
 static void LPTIM1_IRQHandler(void)
@@ -194,21 +191,22 @@ void lp_ticker_set_interrupt(timestamp_t timestamp)
     LptimHandle.Instance = LPTIM1;
     irq_handler = (void (*)(void))lp_ticker_irq_handler;
 
+    __HAL_LPTIM_CLEAR_FLAG(&LptimHandle, LPTIM_FLAG_CMPOK);
+    __HAL_LPTIM_COMPARE_SET(&LptimHandle, 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 */
     while (__HAL_LPTIM_GET_FLAG(&LptimHandle, LPTIM_FLAG_CMPOK) == RESET) {
     }
 
-    __HAL_LPTIM_CLEAR_FLAG(&LptimHandle, LPTIM_FLAG_CMPOK);
-    __HAL_LPTIM_CLEAR_FLAG(&LptimHandle, LPTIM_FLAG_CMPM);
-    __HAL_LPTIM_COMPARE_SET(&LptimHandle, timestamp);
+    lp_ticker_clear_interrupt();
 
     NVIC_EnableIRQ(LPTIM1_IRQn);
 }
 
 void lp_ticker_fire_interrupt(void)
 {
     lp_Fired = 1;
+    irq_handler = (void (*)(void))lp_ticker_irq_handler;
     NVIC_SetPendingIRQ(LPTIM1_IRQn);
     NVIC_EnableIRQ(LPTIM1_IRQn);
 }
@@ -217,9 +215,6 @@ void lp_ticker_disable_interrupt(void)
 {
     NVIC_DisableIRQ(LPTIM1_IRQn);
     LptimHandle.Instance = LPTIM1;
-    /* Waiting last write operation completion */
-    while (__HAL_LPTIM_GET_FLAG(&LptimHandle, LPTIM_FLAG_CMPOK) == RESET) {
-    }
 }
 
 void lp_ticker_clear_interrupt(void)