slip: remove redundant check slip_devs for NULL

As slip_devs is initialized on module load stage there is no reason to
check it for NULL anywhere instead of the deinitialization routine because
if we can't get enough memory on startup we don't run at all.

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