netifd: fix a regression related to ppp + multiple updates (ipv4 + ipv6)

Fixes #15504

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

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@40443 3c298f89-4303-0410-b956-a3cf2f4a3e73