firewall: allow DHCPv6 traffic to/from fc00::/6 instead of fe80::/10

There is no RFC requirement that DHCPv6 servers must reply with a link local
address and some ISP servers in the wild appear to using addresses in the ULA
range to send DHCPv6 offers.

Signed-off-by: Jo-Philipp Wich <jow@openwrt.org>

SVN-Revision: 47048