nft-qos: simplify ifname retrieval

network_get_device should be enough, and since https://git.openwrt.org/?p=openwrt/openwrt.git;a=commitdiff;h=4b9a67362d70c544b85078b8d5c661f43f7472d9


uci network config interface sections use 'device' instead of 'ifname',
rendering the fallback useless

Signed-off-by: Etienne Champetier <champetier.etienne@gmail.com>