slip: remove dead code within the slip initialization

This following code contains a dead "if (dev).." block:
        ...
        for (i = 0; i < slip_maxdev; i++) {
                dev = slip_devs[i];
                if (dev == NULL)
                        break;
        }
        /* Sorry, too many, all slots in use */
        if (i >= slip_maxdev)
                return NULL;

        if (dev) {
                sl = netdev_priv(dev);
                if (test_bit(SLF_INUSE, &sl->flags)) {
                        unregister_netdevice(dev);
                        dev = NULL;
                        slip_devs[i] = NULL;
                }
         }
        ...
The reason is that the code starting with "if (dev).." is never called as
when we found an empty slot (dev == NULL) we break the loop and "if (dev).."
not works eiter the loop ends and we get out with "i >= slip_maxdev".

Signed-off-by: Matvejchikov Ilya <matvejchikov@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>