comgt: in directip, derive the wwan ifname automatically instead of specifying it in the config

Signed-off-by: Felix Fietkau <nbd@openwrt.org>

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@42669 3c298f89-4303-0410-b956-a3cf2f4a3e73
comgt: fireup a dhcpv6-client for directip as well

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@42670 3c298f89-4303-0410-b956-a3cf2f4a3e73