luci-app-mwan3: honor dynamic interface to get gateway ip

If a logical interface setup and adds in the protocol handler a dynamic
interface then the gateway is configured in the dynamic interface and the
setting up logical interface does not have a gateway specified.
To fix this check first if a dynamic interface is present and use this
gateway ip if found and if no dynamich interface is set then check for a
gateway in the logical interface.

Signed-off-by: Florian Eckert <fe@dev.tdt.de>