slcan: fix ldisc->open retval

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

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