net: dsa: rtl8366: Fix VLAN semantics

The RTL8366 would not handle adding new members (ports) to
a VLAN: the code assumed that ->port_vlan_add() was only
called once for a single port. When intializing the
switch with .configure_vlan_while_not_filtering set to
true, the function is called numerous times for adding
all ports to VLAN1, which was something the code could
not handle.

Alter rtl8366_set_vlan() to just |= new members and
untagged flags to 4k and MC VLAN table entries alike.
This makes it possible to just add new ports to a
VLAN.

Put in some helpful debug code that can be used to find
any further bugs here.

Cc: DENG Qingfang <dqfext@gmail.com>
Cc: Mauri Sandberg <sandberg@mailfence.com>
Reviewed-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: David S. Miller <davem@davemloft.net>