dhcp6: pd: T4447: bugfix sla-id limits (must be greater then 128

The sla-id parameter of DHCPv6 prefix delegations is limited to 128. While this
is enough to use all /64 subnets of a /57 prefix, with a /56 prefix that is no
longer sufficient.

Increased sla-id length tp 65535 so one could delegate an entire /48.

(cherry picked from commit 283276d457a09c100416c0d4ffccd4f94ccd2540)