Reduce the flash latency to 2WS as that is sufficient for the 84MHz clock.
Can't use HSE clock on the Nucleo board as it's not populated.

The board as supplied does not provide any external clock to the F4.
It could supply an 8MHz clock from the STLINK part but, by default, it
isn't configured to do that.

So just use the HSI instead of the HSE - it won't be good for USB or fast
CAN bit rates but should be fine for bit banging, etc.