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

@@ -161,9 +161,11 @@ FLASH_Status FLASH_ProgramHalfWord(uint32_t Address, uint16_t Data) {
  * @retval None
  */
 void FLASH_Unlock(void) {
-    /* Authorize the FPEC Access */
-    FLASH->KEYR = FLASH_KEY1;
-    FLASH->KEYR = FLASH_KEY2;
+    if (FLASH->CR & FLASH_CR_LOCK) {
+        /* Authorize the FPEC Access */
+        FLASH->KEYR = FLASH_KEY1;
+        FLASH->KEYR = FLASH_KEY2;
+    }
 }
 
 /**