slcan: remove obsolete code in slcan initialisation

This patch removes obsolete code in the initialisation/creation of
slcan devices.

It follows the suggested cleanups from Ilya Matvejchikov in
drivers/net/slip.c that where recently applied to net-next-2.6:

- slip: remove dead code within the slip initialization
- slip: remove redundant check slip_devs for NULL

Signed-off-by: Oliver Hartkopp <socketcan@hartkopp.net>
Signed-off-by: David S. Miller <davem@davemloft.net>