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

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'.

These scripts use '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.

Follow P/R:
 #14431

Signed-off-by: Coia Prant <coiaprant@gmail.com>