net: dsa: rtl8366: Properly clear member config

When removing a port from a VLAN we are just erasing the
member config for the VLAN, which is wrong: other ports
can be using it.

Just mask off the port and only zero out the rest of the
member config once ports using of the VLAN are removed
from it.

Reported-by: Florian Fainelli <f.fainelli@gmail.com>
Fixes: d8652956

 ("net: dsa: realtek-smi: Add Realtek SMI driver")
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>