flash_stm32: don't unlock flash if already unlocked (#13841)

On stm32f4 after reboot from DFU it was observed that the flash is
already unlocked. In that case, attempting to unlock it again causes a
data abort.

tmk_core/common/chibios/flash_stm32.c

@@ -161,10 +161,12 @@ FLASH_Status FLASH_ProgramHalfWord(uint32_t Address, uint16_t Data) {
  * @retval None
  */
 void FLASH_Unlock(void) {
+    if (FLASH->CR & FLASH_CR_LOCK) {
         /* Authorize the FPEC Access */
         FLASH->KEYR = FLASH_KEY1;
         FLASH->KEYR = FLASH_KEY2;
     }
+}
 
 /**
  * @brief  Locks the FLASH Program Erase Controller.