scripts: dhcpv6: don't report custom ip6class if default

scripts: dhcpv6: don't report custom ip6class if default

odhcp6c returns delegated prefixes with a "class=<IAID>" tag if a custom
IAID was specified in the -P argument. This tag must then be used for any
prefix filter ("network.<interface>.ip6class") instead of the uplink i/f
name.

OpenWrt by default uses the first eight digits of the uplink i/f name's MD5
hash as IAID. Don't report a custom "class" tag for this default IAID.

Fixes: f08cd5ce5d66d76fa6e282d6e3f29bab9cdf8023
Signed-off-by: Shine <4c.fce2@proton.me>
Tested-by: Goetz Goerisch <ggoerisch@gmail.com>
Link: https://github.com/openwrt/openwrt/pull/23989
Signed-off-by: Robert Marko <robimarko@gmail.com>