net: fec: memory corruption caused by err_enet_mii_probe error path

Hello!

There is an issue with the error path of fec_enet_mii_probe in
fec_enet_open(struct net_device *ndev) which leads to random memory
corruption.

In open() the buffers are initialized:
https://github.com/torvalds/linux/blob/v5.10/drivers/net/ethernet/freescale/fec_main.c#L3001

Then fec_restart will start the DMA engines.
https://github.com/torvalds/linux/blob/v5.10/drivers/net/ethernet/freescale/fec_main.c#L3006

Now if fec_enet_mii_probe fails (e.g. phy did not respond via mii) the
err_enet_mii_probe error path will be used
https://github.com/torvalds/linux/blob/v5.10/drivers/net/ethernet/freescale/fec_main.c#L3031

err_enet_mii_probe:
fec_enet_free_buffers(ndev);
err_enet_alloc:
fec_enet_clk_enable(ndev, false);
clk_enable:
pm_runtime_mark_last_busy(&fep->pdev->dev);
pm_runtime_put_autosuspend(&fep->pdev->dev);
pinctrl_pm_select_sleep_state(&fep->pdev->dev);
return ret;

This error path frees the DMA buffers, BUT as far I could see it does
not stop the DMA engines.
=> open() fails => frees buffers => DMA still active => MAC receives
network packet => DMA starts => random memory corruption (use after
free) => random kernel panics

So maybe fec_stop() as counterpart to fec_restart() is missing before
freeing the buffers?
err_enet_mii_probe:
fec_stop(ndev);
fec_enet_free_buffers(ndev);

Issue happend with 5.10.83 and should also also happen with current master.