traffic-control: Obey $NET_INTERFACE in traffic-control-tcdemo test

Don’t hard-code the traffic controlled interface to eth0, or the
traffic-control-tcdemo test will fail on devices where eth0 is not the
main network interface.

Bug: https://bugs.apertis.org/show_bug.cgi?id=328
Differential Revision: https://phabricator.apertis.org/D431


Signed-off-by: Philip Withnall <philip.withnall@collabora.co.uk>
Reviewed-by: Sjoerd Simons <sjoerd.simons@collabora.co.uk>