gluon-core: only restore WAN proto on upgrade (#2576)

Only restore the netifd proto for the WAN bridge in case the upgrade is
done from an older Gluon version.

For DSL targets, OpenWrt defaults the WAN proto to pppoe, while Gluon
uses the Ethernet ports for WAN. When unconditionally preserving the WAN
proto, pppoe is carried over to Gluon's network config.

Signed-off-by: David Bauer <mail@david-bauer.net>