libdsl: add function to retrieve ifname for dsl interface
For dslmngr to be able to retrieve if stats counters it needs to know which interface dsl is running on. We assume adsl uses atm-device and vdsl/fast use ptm-device from uci.
For dslmngr to be able to retrieve if stats counters it needs to know which interface dsl is running on. We assume adsl uses atm-device and vdsl/fast use ptm-device from uci.