Correctly check PLL readiness

src/main.c

@@ -51,7 +51,7 @@ static void system_clock_init(void) {
 
   // Wait indefinitely for PLL to be ready
   // TODO indicate error/timeout somehow?
-  while (!(RCC->CR & RCC_CR_HSERDY_READY));
+  while (!(RCC->CR & RCC_CR_PLLRDY_LOCKED));
 
   // Enable caching of instructions and data
   FLASH->ACR |= FLASH_ACR_DCEN_ENABLE;