net: dsa: rtl8366rb: Always treat VLAN 0 as untagged

VLAN 0 shall always be treated as untagged, as per example
from other drivers (I guess from the spec).

Cc: Vladimir Oltean <olteanv@gmail.com>
Cc: Mauri Sandberg <sandberg@mailfence.com>
Cc: Alvin Šipraga <alsi@bang-olufsen.dk>
Cc: Florian Fainelli <f.fainelli@gmail.com>
Cc: DENG Qingfang <dqfext@gmail.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
---
ChangeLog v4->v5:
- No changes just resending with the rest of the
  patches.
ChangeLog v1->v4:
- New patch after noting that other drivers always sets VLAN 0
  as untagged.