slip: fix wrong SLIP6 ifdef-endif placing

SLIP6 have nothing to do with CSLIP so placing a block of
SLIP6-related code within a CSLIP ifdef-endif block is incorrect.

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