net: dsa: rtl8366rb: Use core filtering tracking

We added a state variable to track whether a certain port
was VLAN filtering or not, but we can just inquire the DSA
core about this.

Cc: Vladimir Oltean <olteanv@gmail.com>
Cc: Mauri Sandberg <sandberg@mailfence.com>
Cc: DENG Qingfang <dqfext@gmail.com>
Cc: Alvin Šipraga <alsi@bang-olufsen.dk>
Cc: Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: David S. Miller <davem@davemloft.net>