NET: slip, fix ldisc->open retval

TTY layer expects 0 if the ldisc->open operation succeeded.

Signed-off-by : Matvejchikov Ilya <matvejchikov@gmail.com>
Acked-by: Oliver Hartkopp <socketcan@hartkopp.net>
Acked-by: Alan Cox <alan@linux.intel.com>

Signed-off-by: David S. Miller <davem@davemloft.net>