The untagged ethernet interface of mt7621 doesn't work

The untagged ethernet interface eth0 worked before, but after this patch https://github.com/openwrt/openwrt/commit/c9262a96d1fc0a7fb1ba0806acb3b7aae00b1ecf, it doesn't work any more. Tagged interface such as eth0.1,eth0.2 still works.