[FS#991] untagged vlan do not work on mt7620

[LEDE Bugs]

The following task has a new comment added:

FS#991 - untagged vlan do not work on mt7620
User who did this - Routing Route (rrt)

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Maybe see if it is like FS#1341 - vlan configuration fails, trying the workaround.
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More information can be found at the following URL:
https://bugs.lede-project.org/index.php?do=details&task_id=991#comment4295