net: dsa: rtl8366: Refactor VLAN/PVID init

The VLANs and PVIDs on the RTL8366 utilizes a "member
configuration" (MC) which is largely unexplained in the
code.

This set-up requires a special ordering: rtl8366_set_pvid()
must be called first, followed by rtl8366_set_vlan(),
else the MC will not be properly allocated. Relax this
by factoring out the code obtaining an MC and reuse
the helper in both rtl8366_set_pvid() and
rtl8366_set_vlan() so we remove this strict ordering
requirement.

In the process, add some better comments and debug prints
so people who read the code understand what is going on.

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>