net/ethernet: ks8851_mll unregister_netdev() before freeing

We added another error condition here, but if we were to hit it then
we need to unregister_netdev() before doing the free_netdev().
Otherwise we would hit the BUG_ON() in free_netdev():

	BUG_ON(dev->reg_state != NETREG_UNREGISTERED);

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Tested-by: Raffaele Recalcati <raffaele.recalcati@bticino.it>
Signed-off-by: David S. Miller <davem@davemloft.net>