ddns-scripts: get l3 device for bind network using curl

If pppoe is used for wan access. script set 'eth1' as interface for curl
call. The correct interface is however 'pppoe-wan'.

The script uses 'network_get_physdev' function to get real device for
bind_network but this is wrong. We need instead the l3_device of the the
logical interface.

In case if we don't use pppoe connection - 'l3_device' is equal to real device.

This was reported by the github user `welderpb` with P/R:
 https://github.com/openwrt/packages/pull/14431



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