stm32: Only use the HSI48 clock when doing "usb clock recovery"

The HSI48 clock on the stm32f042 isn't as accurate as the main HSI
clock.  So, only use the HSI48 clock when using USB to synchronize
that clock.

Signed-off-by: Kevin O'Connor <kevin@koconnor.net>

src/stm32/stm32f0.c

@@ -211,7 +211,8 @@ armcm_main(void)
     FLASH->ACR = (1 << FLASH_ACR_LATENCY_Pos) | FLASH_ACR_PRFTBE;
 
     // Configure main clock
-    if (CONFIG_MACH_STM32F042 && CONFIG_STM32_CLOCK_REF_INTERNAL)
+    if (CONFIG_MACH_STM32F042 && CONFIG_STM32_CLOCK_REF_INTERNAL
+        && CONFIG_USBSERIAL)
         hsi48_setup();
     else
         pll_setup();