fix: correctly deinit rcc before hse sw

main.c

@@ -29,13 +29,21 @@
 struct netif g_netif;
 static volatile int g_httpd_is_initialized = 0;
 
-int clock_init(void);
 void led_init(void);
 void lwip_stack_init(void);
 
 static void set_sysclk_to_120mhz_from_hse(void) {
-  volatile uint32_t startup_counter = 0;
+  uint32_t startup_counter = 0;
 
+  RCC->INTR = 0x009F0000;  // clear PLL, CSSC, HSE, HSI and LSI ready flags.
+  // switch processor back to HSI so we don't eat dirt.
+  RCC->CFGR0 = 0;
+  // disable PLL so we can play with it.
+  RCC->CTLR &= ~RCC_PLLON;
+  // not sure why, need to reset here, otherwise PLLXTPRE is set.
+  RCC->CFGR0 = RCC_PLLSRC;
+
+  // enable HSE
   RCC->CTLR |= RCC_HSEON;
 
   do {