dhcp: Fix typo in dhclient-script

- Fixed dhclient-script typo causing ISC DHCPv6 client to
execute ifup pre-down scripts also while renew, when the
ipv6 address did not changed (bnc#690859).

Fixed bugs
bnc#690859
DHCPv6 client performs ifdown/ifup at each lease renewal
Selected Binaries
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